AP. Sequence a,a+d,a+2d,a+3d,β¦ with first term a and common difference d.
Formulas.
- nth term: unβ=a+(nβ1)d.
- Sum of n terms: Snβ=2nβ(2a+(nβ1)d)=2nβ(a+l), where l is last term.
Recognising an AP. Check: u2ββu1β=u3ββu2β=d.
Example. a=5, d=3. u20β=5+19β
3=62. S20β=10(10+57)=670.
Cambridge tip. Identify a and d first, then choose formula.