What is the Sine Rule in Trigonometry and when is it used?

The Sine Rule, also known as the Law of Sines, is a formula used in trigonometry to find unknown lengths or angles in any triangle, not just right-angled ones. It is particularly useful in solving non-right-angled triangles. The rule states that the ratio of the length of a side to the sine of its opposite angle is constant for all three sides of the triangle. Mathematically, it is expressed as a/sin(A) = b/sin(B) = c/sin(C), where a, b, and c are the sides, and A, B, and C are the respective opposite angles.

How do you apply the Sine Rule to solve a triangle problem?

To apply the Sine Rule, first identify the known sides and angles in the triangle. If you have two angles and one side (AAS or ASA) or two sides and a non-included angle (SSA), you can use the Sine Rule. Set up the equation using the formula a/sin(A) = b/sin(B) = c/sin(C). Substitute the known values into the equation and solve for the unknown side or angle. This method is part of the Cambridge IGCSE Mathematics curriculum, which emphasizes understanding and applying trigonometric principles.

What are the limitations of the Sine Rule in solving triangles?

The Sine Rule is not applicable for right-angled triangles, where the Pythagorean theorem or basic trigonometric ratios are more suitable. Additionally, when using the Sine Rule with the SSA condition, there can be two possible solutions (the ambiguous case), one solution, or no solution, depending on the given values. Understanding these limitations is crucial for Cambridge IGCSE students to correctly apply the Sine Rule in various scenarios.

Can the Sine Rule be used to find the area of a triangle?

Yes, the Sine Rule can indirectly help find the area of a triangle. Once you have determined all sides or angles using the Sine Rule, you can use the formula for the area of a triangle: Area = 1/2 * a * b * sin(C), where a and b are two sides, and C is the included angle. This approach is part of the broader trigonometric applications taught in the Cambridge IGCSE Mathematics syllabus.

How does the Sine Rule relate to the Law of Cosines?

The Sine Rule and the Law of Cosines are both used to solve triangles, but they apply to different scenarios. The Sine Rule is used when you have two angles and a side (AAS or ASA) or two sides and a non-included angle (SSA). In contrast, the Law of Cosines is used when you have two sides and the included angle (SAS) or all three sides (SSS). Understanding when to use each rule is essential for solving trigonometry problems in the Cambridge IGCSE Mathematics curriculum.

Why is "Mismatching sides and angles in the sine rule" a common mistake in Sine Rule?

Why it happens: Forgetting that each side pairs with its OPPOSITE angle. How to avoid it: a goes with A, b goes with B. Match by capital/lowercase letter.

Why is "Missing the obtuse possibility in the ambiguous case" a common mistake in Sine Rule?

Why it happens: Calculator only returns the acute angle. How to avoid it: When given SSA, check 180 - ^-1() as well. Source: 0580/42 — recurring

Why is "Using sine rule when cosine rule is needed" a common mistake in Sine Rule?

Why it happens: Default to sine rule. How to avoid it: Sine rule requires a side and its opposite angle. If you have SAS or SSS, use COSINE rule.

Cambridge IGCSE Mathematics (0580)

Sine Rule

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Short Notes - Sine Rule

Detailed Study Notes

Detailed notes on Trigonometry for Cambridge IGCSE Mathematics, covering key concepts, explanations, examples, and exam-focused revision points.

Sine Rule — Cambridge IGCSE 0580 Maths Extended (2026)

$\dfrac{a}{\sin A} = \dfrac{b}{\sin B} = \dfrac{c}{\sin C}$ . Use it when you know an angle-side OPPOSITE pair plus one more piece. The non-right-angled triangle workhorse.

At a glance

Sine rule: $\dfrac{a}{\sin A} = \dfrac{b}{\sin B} = \dfrac{c}{\sin C}$ .
Naming convention: side $a$ is OPPOSITE angle $A$ .
Use when you have an opposite angle-side PAIR plus one extra side or angle.
Two cases: AAS (find a side) and ASS (find an angle).
ASS — be alert to the AMBIGUOUS case (two possible angles).
Calculator in DEGREE mode.

What you’ll learn

Mapped to the Cambridge IGCSE 0580 syllabus (2025-2027).

E8.5 — Use the sine rule to solve problems involving the lengths and angles of any triangle.

The sine rule

$\dfrac{a}{\sin A} = \dfrac{b}{\sin B} = \dfrac{c}{\sin C}$ .

For any triangle (not just right-angled) with sides $a, b, c$ opposite angles $A, B, C$ : $\dfrac{a}{\sin A} = \dfrac{b}{\sin B} = \dfrac{c}{\sin C}.$

Naming convention. Lower case $a, b, c$ are the SIDES. Upper case $A, B, C$ are the OPPOSITE ANGLES. So side $a$ is across from angle $A$ .

Each side is named with the lower-case letter of the angle it faces.

The shared height gives b sin A = a sin B, which rearranges to a/sin A = b/sin B.

When to use. When you have an angle-and-its-opposite-side pair, plus ONE extra piece:

Two angles + one side (AAS): find another side.
Two sides + a non-included angle (ASS): find another angle.

Worked. Triangle with angle $A = 50\degree$ , side $a = 8\,\text{cm}$ , angle $B = 70\degree$ . Find side $b$ .

$\dfrac{8}{\sin 50\degree} = \dfrac{b}{\sin 70\degree}$ .
$b = \dfrac{8 \sin 70\degree}{\sin 50\degree} \approx 9.81\,\text{cm}$ .

$\dfrac{a}{\sin A} = \dfrac{b}{\sin B} = \dfrac{c}{\sin C}$ .
Side $a$ opposite angle $A$ .
Use when you have an opposite pair + one more.
Calculator in DEG mode.

Finding a missing side (AAS)

Two angles and one side known — sum of angles gives the third angle, then sine rule for the missing side.

Strategy.

If you have two angles, find the third using $A + B + C = 180\degree$ .
Identify the angle-side OPPOSITE pair you know.
Set up the sine rule with the unknown side over its opposite angle.
Cross-multiply to solve.

Worked. Triangle with angles $A = 35\degree, B = 80\degree$ , side $b = 12\,\text{cm}$ opposite $B$ . Find side $a$ .

$\dfrac{a}{\sin 35\degree} = \dfrac{12}{\sin 80\degree}$ .
$a = \dfrac{12 \sin 35\degree}{\sin 80\degree} \approx 6.99\,\text{cm}$ .

Two angles and the side opposite one of them → the sine rule gives the side opposite the other.

Tip. ALWAYS state which angle goes with which side. Cambridge marks the set-up — even if your final number is off, you can score "method" marks.

Find the third angle first if you have two.
Use side / sin(opposite angle) on both halves.
Cross-multiply, then divide by the trig value.

Finding a missing angle (ASS)

Two sides + a non-included angle known. Apply sine rule, then $\sin^{-1}$ . Watch for the ambiguous case.

Strategy.

Identify the angle-and-opposite-side pair you know.
Set up the sine rule with the unknown angle over its opposite side.
Cross-multiply, isolate $\sin\theta$ , then take $\sin^{-1}$ .

Worked. Triangle with side $a = 9\,\text{cm}$ opposite angle $A$ , side $b = 14\,\text{cm}$ opposite angle $B$ , and $A = 38\degree$ . Find $B$ .

$\dfrac{9}{\sin 38\degree} = \dfrac{14}{\sin B}$ .
$\sin B = \dfrac{14 \sin 38\degree}{9} \approx 0.9576$ .
$B = \sin^{-1}(0.9576) \approx 73.3\degree$ .

Ambiguous case. When you compute $\sin B$ and there are TWO possible angles ( $B$ and $180\degree - B$ ), check whether both are geometrically possible. The supplementary angle is also a valid sine value. Usually the diagram or context tells you which is correct.

Worked (ambiguous). $\sin B = 0.866 \Rightarrow B = 60\degree$ OR $B = 180\degree - 60\degree = 120\degree$ . Both have sine $0.866$ .

An angle and its supplement give the same sine — always check whether the obtuse option fits the triangle.

If $A + B$ would exceed $180\degree$ , the obtuse $B$ is impossible. Otherwise, both might be valid — check the diagram.

Set up sine rule with unknown angle over its side.
Take $\sin^{-1}$ to find the angle.
AMBIGUOUS case: the supplementary angle ( $180\degree - \theta$ ) is also a valid sine.
Check via the diagram or angle sum.

When to use sine vs cosine rule

Sine rule: opposite angle-side pair known. Cosine rule: no such pair (SAS or SSS).

Decision flow.

Right-angled triangle? → SOH CAH TOA.
Have an angle and its OPPOSITE side? → sine rule.
Don't have an opposite pair (only SAS or SSS) → cosine rule.

Examples.

Given	Tool
Right angle + 2 sides	SOH CAH TOA / Pythagoras
2 angles + 1 side (AAS)	Sine rule
2 sides + non-included angle (ASS)	Sine rule (mind ambiguity)
2 sides + included angle (SAS)	Cosine rule (find third side)
3 sides (SSS)	Cosine rule (find an angle)

Tip. The TWO sides matter — does one of them face an angle you ALSO know? Then sine rule. Otherwise cosine rule.

Right-angled? SOH CAH TOA.
Opposite angle-side pair? Sine rule.
SAS or SSS? Cosine rule.

Quick recap

$\dfrac{a}{\sin A} = \dfrac{b}{\sin B} = \dfrac{c}{\sin C}$ .
Use when you have an opposite angle-side pair + one extra.
AAS: find the third angle, then a missing side.
ASS: find a missing angle (watch ambiguous case).
Calculator in DEG mode.
Right-angled? SOH CAH TOA. SAS/SSS? Cosine rule.

Memorise this

Verbatim phrases and definitions Cambridge mark schemes credit.

Sine rule — $\dfrac{a}{\sin A} = \dfrac{b}{\sin B} = \dfrac{c}{\sin C}$ for any triangle.
AAS — angle-angle-side: find a missing side.
ASS — angle-side-side: find a missing angle (ambiguous case possible).
Ambiguous case — when the sine equation gives two valid angles ( $\theta$ and $180\degree - \theta$ ).

How it’s examined

Sine rule appears every Paper 4 (4-6 marks) and most Paper 2s (3-4 marks). Cambridge often combines it with bearings, area-of-triangle, or compound-shape questions. Examiner reports flag two errors: (i) wrong angle-side pairing in the formula set-up, (ii) missing the ambiguous case when finding obtuse angles.

Sources: Cambridge IGCSE Mathematics 0580 syllabus 2025-2027 (E8.5); 0580/42 Oct/Nov 2024 — Q15 (sine rule + bearings); 0580 Examiner Reports 2022-2024. Last reviewed 2026-05-05.

Take this whole topic with you

Step-by-step worked examples — Sine Rule

Step-by-step solutions to past-paper-style questions on sine rule, written exactly the way a tutor would explain them at the board.

Question type:

Question patterns to master — Sine Rule

Almost every sine rule exam question is one of these shapes. Learn to spot each one and you will always know how to start.

Direct calculation

Recognise it by

A non-right-angled triangle where a side is paired with its opposite angle (AAS / ASA / SSA), with a single find the side / angle instruction.

How to approach it

Pair each side with its opposite angle and write $\dfrac{a}{\sin A}=\dfrac{b}{\sin B}$ . Find the third angle from the triangle sum if needed, then solve. For an angle, finish with $\sin^{-1}$ .

Common trap

Pairing a side with the wrong (non-opposite) angle. Examiner reports flag this in AAS questions — match by capital and lowercase letter.

Word problem

Recognise it by

A navigation context — ports, boats, bearings in degrees — with no triangle drawn and no angle stated directly inside it.

How to approach it

Sketch the triangle, use north lines and back-bearings ( $\pm180\degree$ ) to compute the interior angles, then apply the sine rule as a direct calculation.

Common trap

Substituting raw bearing values (e.g. $070\degree$ ) instead of the included interior angle. Examiner reports note candidates skip the back-bearing step.

Multi-step problem

Recognise it by

An SSA configuration that yields two valid triangles — the question asks for both values of the angle and both corresponding sides.

How to approach it

Solve for the acute angle, then take $180\degree-$ that for the obtuse alternative. Check each against the triangle sum, then complete both cases separately.

Common trap

Quoting one case only, or not checking that both satisfy the angle sum. Examiner reports note marks split between recognising the ambiguity and completing each case.

Show that / prove

Recognise it by

The words show that / prove a length relationship — the answer is given and you must derive it.

How to approach it

Start from the sine rule, substitute the given angle relationship, apply identities such as $\sin 2A=2\sin A\cos A$ , and simplify step by step to the stated result.

Common trap

Cancelling $\sin A$ without stating it is non-zero. Examiner reports note each algebraic step must be shown, with the cancellation justified.

1Find a side using the sine rule
ExtendedDirect calculation• Adapted from 0580/42 May/Jun 2024 Q15• find side
Question
In triangle $ABC$ , $\angle A = 50\degree$ , $\angle B = 60\degree$ , and $a = 8$ cm. Find $b$ .
Step-by-step solution
1. Step 1
  Use $\dfrac{a}{\sin A} = \dfrac{b}{\sin B}$ .
  $\dfrac{8}{\sin 50\degree} = \dfrac{b}{\sin 60\degree}$
2. Step 2
  Solve for $b$ .
  $b = \dfrac{8 \sin 60\degree}{\sin 50\degree} \approx 9.04\,\text{cm}$
Answer
$b \approx 9.04\,\text{cm}$
2Find an angle using the sine rule
ExtendedDirect calculation• find angle
Question
In triangle $ABC$ , $a = 7$ cm, $b = 10$ cm, $\angle A = 35\degree$ . Find $\angle B$ .
Step-by-step solution
1. Step 1
  $\dfrac{\sin A}{a} = \dfrac{\sin B}{b}$ .
  $\sin B = \dfrac{10 \sin 35\degree}{7}$
2. Step 2
  Compute and apply $\sin^{-1}$ .
  $\sin B \approx 0.819 \Rightarrow B \approx 55.0\degree$
Answer
$\angle B \approx 55.0\degree$
3Ambiguous case
ExtendedDirect calculation• ambiguous
Question
In triangle $ABC$ , $a = 8$ , $b = 12$ , $\angle A = 30\degree$ . Find $\angle B$ .
Step-by-step solution
1. Step 1
  $\sin B = \dfrac{12 \sin 30\degree}{8} = 0.75$ .
  $\sin B = 0.75$
2. Step 2
  $B = \sin^{-1}(0.75) \approx 48.6\degree$ OR $B \approx 180\degree - 48.6\degree = 131.4\degree$ (since $\sin$ is positive in both quadrants).
3. Step 3
  Both work geometrically. Always state both possibilities unless the diagram rules one out.
Answer
$\angle B \approx 48.6\degree \text{ or } 131.4\degree$
Examiner tip
When given two sides and a non-included angle, ALWAYS check for the second valid angle.
4Find a side given two angles and a side
CoreDirect calculation• Adapted from 0580/22 May/Jun 2023 Q14• find side, AAS
Question
In triangle $PQR$ , $\angle P = 42\degree$ , $\angle Q = 78\degree$ and $PQ = 9$ cm. Find $QR$ .
Step-by-step solution
1. Step 1
  Find the third angle first using the triangle sum.
  $\angle R = 180\degree - 42\degree - 78\degree = 60\degree$
2. Step 2
  Side $PQ$ is opposite $\angle R$ ; side $QR$ is opposite $\angle P$ . Apply the sine rule.
  $\dfrac{QR}{\sin 42\degree} = \dfrac{9}{\sin 60\degree}$
3. Step 3
  Solve.
  $QR = \dfrac{9 \sin 42\degree}{\sin 60\degree} \approx 6.96\,\text{cm}$
Answer
$QR \approx 6.96$ cm
Examiner tip
Always pair each side with the angle directly opposite it. Pairing $QR$ with $\angle Q$ is the most common slip in AAS problems.
5Sine rule with bearings
ExtendedWord problem• Adapted from 0580/42 Feb/Mar 2024 Q13• bearings, find side
Question
From port $A$ , the bearing of port $B$ is $070\degree$ and the bearing of buoy $C$ is $120\degree$ . From port $B$ , the bearing of $C$ is $200\degree$ . If $AB = 24$ km, find $AC$ .
Step-by-step solution
1. Step 1
  Angle at $A$ : $\angle BAC = 120\degree - 70\degree = 50\degree$ .
2. Step 2
  Angle at $B$ : the back-bearing of $A$ from $B$ is $250\degree$ , so $\angle ABC = 250\degree - 200\degree = 50\degree$ .
3. Step 3
  Third angle: $\angle ACB = 180\degree - 50\degree - 50\degree = 80\degree$ .
4. Step 4
  Apply the sine rule. $AC$ is opposite $\angle B$ , $AB$ is opposite $\angle C$ .
  $\dfrac{AC}{\sin 50\degree} = \dfrac{24}{\sin 80\degree}$
5. Step 5
  Solve.
  $AC = \dfrac{24 \sin 50\degree}{\sin 80\degree} \approx 18.67\,\text{km}$
Answer
$AC \approx 18.7$ km
Examiner tip
Set up the angles inside the triangle carefully — many candidates use the raw bearing values instead of computing the included angle.
6Real-world: boat journey
ExtendedWord problem• real-world, navigation
Question
A boat sails from $X$ to $Y$ , a distance of $32$ km. The bearing of $Y$ from $X$ is $050\degree$ . From $Y$ it then sails to $Z$ on a bearing of $140\degree$ . The bearing of $Z$ from $X$ is $090\degree$ . Find the distance $YZ$ .
Step-by-step solution
1. Step 1
  At $X$ , the angle between $XY$ and $XZ$ is $90\degree - 50\degree = 40\degree$ , so $\angle YXZ = 40\degree$ .
2. Step 2
  At $Y$ , the back-bearing of $X$ is $230\degree$ . The angle $\angle XYZ = 230\degree - 140\degree = 90\degree$ .
3. Step 3
  Third angle: $\angle XZY = 180\degree - 40\degree - 90\degree = 50\degree$ .
4. Step 4
  Apply the sine rule with $YZ$ opposite $\angle X$ and $XY$ opposite $\angle Z$ .
  $\dfrac{YZ}{\sin 40\degree} = \dfrac{32}{\sin 50\degree}$
5. Step 5
  Solve.
  $YZ = \dfrac{32 \sin 40\degree}{\sin 50\degree} \approx 26.85\,\text{km}$
Answer
$YZ \approx 26.9$ km
7Find an angle opposite the larger side
ExtendedDirect calculation• Adapted from 0580/42 Oct/Nov 2022 Q12• find angle
Question
In triangle $LMN$ , $LM = 12$ cm, $MN = 9$ cm and $\angle MNL = 70\degree$ . Find $\angle MLN$ .
Step-by-step solution
1. Step 1
  Side $LM$ is opposite $\angle N$ ; side $MN$ is opposite $\angle L$ . Apply the sine rule.
  $\dfrac{\sin L}{9} = \dfrac{\sin 70\degree}{12}$
2. Step 2
  Solve.
  $\sin L = \dfrac{9 \sin 70\degree}{12} \approx 0.7046$
3. Step 3
  $L = \sin^{-1}(0.7046) \approx 44.8\degree$ . The obtuse alternative ( $135.2\degree$ ) is rejected because the angles must sum to $180\degree$ and $135.2\degree + 70\degree > 180\degree$ .
Answer
$\angle MLN \approx 44.8\degree$
Examiner tip
Always check whether the obtuse alternative is geometrically possible by adding to the given angle. The 2022 mark scheme allows either form of working but requires a single valid answer in this configuration.
8A* triangle with non-included angle
ChallengeMulti-step problem• Adapted from 0580/42 May/Jun 2023 Q22• ambiguous, challenge
Question
Triangle $DEF$ has $DE = 14$ cm, $DF = 10$ cm and $\angle DEF = 38\degree$ . Find the two possible values of $\angle DFE$ and the corresponding values of $EF$ .
Step-by-step solution
1. Step 1
  Apply the sine rule: $\dfrac{\sin F}{14} = \dfrac{\sin 38\degree}{10}$ .
  $\sin F = \dfrac{14 \sin 38\degree}{10} \approx 0.8620$
2. Step 2
  Two solutions: $F_{1} = \sin^{-1}(0.8620) \approx 59.6\degree$ and $F_{2} = 180\degree - 59.6\degree = 120.4\degree$ . Both are geometrically possible since $59.6\degree + 38\degree < 180\degree$ and $120.4\degree + 38\degree < 180\degree$ .
3. Step 3
  Case 1: $\angle D = 180\degree - 38\degree - 59.6\degree = 82.4\degree$ .
  $EF = \dfrac{10 \sin 82.4\degree}{\sin 38\degree} \approx 16.10\,\text{cm}$
4. Step 4
  Case 2: $\angle D = 180\degree - 38\degree - 120.4\degree = 21.6\degree$ .
  $EF = \dfrac{10 \sin 21.6\degree}{\sin 38\degree} \approx 5.98\,\text{cm}$
Answer
$\angle DFE \approx 59.6\degree$ with $EF \approx 16.1$ cm, OR $\angle DFE \approx 120.4\degree$ with $EF \approx 5.98$ cm
Examiner tip
A* candidates must show that both cases satisfy the triangle inequality before quoting both pairs. Marks are split between recognising the ambiguity and completing each case.
9A* prove a length relationship
ChallengeShow that / prove• proof, challenge
Question
In triangle $ABC$ , $\angle B = 2\angle A$ . Use the sine rule to show that $b = \dfrac{2a\cos A \sin A}{\sin A} = 2a\cos A$ .
Step-by-step solution
1. Step 1
  By the sine rule, $\dfrac{a}{\sin A} = \dfrac{b}{\sin B}$ .
  $b = \dfrac{a \sin B}{\sin A}$
2. Step 2
  Substitute $\angle B = 2A$ and use the double-angle identity $\sin 2A = 2\sin A \cos A$ .
  $b = \dfrac{a \cdot 2\sin A \cos A}{\sin A}$
3. Step 3
  Cancel $\sin A$ (which is non-zero in a valid triangle).
  $b = 2a\cos A$
Answer
$b = 2a\cos A$
Examiner tip
0580 occasionally asks for a short trig proof — show each algebraic step. Cancelling $\sin A$ without stating that it is non-zero is a common omission.
10Real-world: ramp angle
CoreDirect calculation• Adapted from 0580/22 Feb/Mar 2024 Q13• real-world
Question
In triangle $RST$ , $\angle R = 28\degree$ , $\angle S = 95\degree$ and $RS = 6.4$ m. Find the length $RT$ .
Step-by-step solution
1. Step 1
  Find the third angle.
  $\angle T = 180\degree - 28\degree - 95\degree = 57\degree$
2. Step 2
  Side $RS$ is opposite $\angle T$ ; side $RT$ is opposite $\angle S$ . Apply the sine rule.
  $\dfrac{RT}{\sin 95\degree} = \dfrac{6.4}{\sin 57\degree}$
3. Step 3
  Solve.
  $RT = \dfrac{6.4 \sin 95\degree}{\sin 57\degree} \approx 7.60\,\text{m}$
Answer
$RT \approx 7.60$ m
Examiner tip
Sine of an obtuse angle ( $95\degree$ ) is still positive and close to $1$ . Quote the calculator value to at least four significant figures before the final rounding.

Key Formulae — Sine Rule

The formulae you need to memorise for sine rule on the Cambridge IGCSE 0580 paper, with every variable defined in plain English and a note on when to use it.

Sine rule
$\dfrac{a}{\sin A} = \dfrac{b}{\sin B} = \dfrac{c}{\sin C}$
$a, b, c$
sides
$A, B, C$
opposite angles
When to use
Use when you know a side and its opposite angle, plus one other piece (side or angle).
Triangle area (two sides + included angle)
$A = \dfrac{1}{2} ab \sin C$
$a, b$
two sides
$C$
included angle
When to use
Area of any triangle when two sides and the angle between them are known.

Key Definitions and Keywords — Sine Rule

Definitions to memorise and the exact keywords mark schemes credit for sine rule answers — sharpened from recent examiner reports for the 2026 0580 sitting.

Non-right-angled triangle
Examiner keyword
Any triangle without a $90\degree$ angle. Solve using the sine or cosine rule.
Included angle
Examiner keyword
The angle between two named sides.
Ambiguous case
Examiner keyword
When the sine rule gives two valid angles (one acute, one obtuse) for a given configuration.

Common Mistakes and Misconceptions — Sine Rule

The traps other students keep falling into on sine rule questions — taken from recent Cambridge IGCSE 0580 examiner reports and mark schemes — and how to avoid them.

✕Mismatching sides and angles in the sine rule
Why it happens
Forgetting that each side pairs with its OPPOSITE angle.
How to avoid it
$a$ goes with $\sin A$ , $b$ goes with $\sin B$ . Match by capital/lowercase letter.
✕Missing the obtuse possibility in the ambiguous case
0580/42 — recurring
Why it happens
Calculator only returns the acute angle.
How to avoid it
When given SSA, check $180\degree - \sin^{-1}(\dots)$ as well.
✕Using sine rule when cosine rule is needed
Why it happens
Default to sine rule.
How to avoid it
Sine rule requires a side and its opposite angle. If you have SAS or SSS, use COSINE rule.

Sine Rule — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Strategy.

Identify the angle-and-opposite-side pair you know.
Set up the sine rule with the unknown angle over its opposite side.
Cross-multiply, isolate $\sin\theta$ , then take $\sin^{-1}$ .

Worked. Triangle with side $a = 9\,\text{cm}$ opposite angle $A$ , side $b = 14\,\text{cm}$ opposite angle $B$ , and $A = 38\degree$ . Find $B$ .

$\dfrac{9}{\sin 38\degree} = \dfrac{14}{\sin B}$ .
$\sin B = \dfrac{14 \sin 38\degree}{9} \approx 0.9576$ .
$B = \sin^{-1}(0.9576) \approx 73.3\degree$ .

Worked (ambiguous). $\sin B = 0.866 \Rightarrow B = 60\degree$ OR $B = 180\degree - 60\degree = 120\degree$ . Both have sine $0.866$ .

An angle and its supplement give the same sine — always check whether the obtuse option fits the triangle.

If $A + B$ would exceed $180\degree$ , the obtuse $B$ is impossible. Otherwise, both might be valid — check the diagram.

Given

Tool

Right angle + 2 sides

SOH CAH TOA / Pythagoras

2 angles + 1 side (AAS)

Sine rule

2 sides + non-included angle (ASS)

Sine rule (mind ambiguity)

2 sides + included angle (SAS)

Cosine rule (find third side)

3 sides (SSS)

Cosine rule (find an angle)

Sine Rule

Short Notes - Sine Rule

Detailed Study Notes

At a glance

What you’ll learn

The sine rule

Finding a missing side (AAS)

Finding a missing angle (ASS)

When to use sine vs cosine rule

Quick recap

Memorise this

How it’s examined

Take this whole topic with you

Question patterns to master — Sine Rule

1Find a side using the sine rule

2Find an angle using the sine rule

3Ambiguous case

4Find a side given two angles and a side

5Sine rule with bearings

6Real-world: boat journey

7Find an angle opposite the larger side

8A* triangle with non-included angle

9A* prove a length relationship

10Real-world: ramp angle

Sine rule

Triangle area (two sides + included angle)

Non-right-angled triangle

Included angle

Ambiguous case

✕Mismatching sides and angles in the sine rule

✕Missing the obtuse possibility in the ambiguous case

✕Using sine rule when cosine rule is needed

Sine Rule — frequently asked questions

Sine Rule

Short Notes - Sine Rule

Detailed Study Notes

At a glance

What you’ll learn

The sine rule

Finding a missing side (AAS)

Finding a missing angle (ASS)

When to use sine vs cosine rule

Quick recap

Memorise this

How it’s examined

Take this whole topic with you

Question patterns to master — Sine Rule

1Find a side using the sine rule

2Find an angle using the sine rule

3Ambiguous case

4Find a side given two angles and a side

5Sine rule with bearings

6Real-world: boat journey

7Find an angle opposite the larger side

8A* triangle with non-included angle

9A* prove a length relationship

10Real-world: ramp angle

Sine rule

Triangle area (two sides + included angle)

Non-right-angled triangle

Included angle

Ambiguous case

✕Mismatching sides and angles in the sine rule

✕Missing the obtuse possibility in the ambiguous case

✕Using sine rule when cosine rule is needed

Sine Rule — frequently asked questions