Pyramid volume: V=31× base area ×h.
The 31 is universal for pyramids/cones — exactly 31 of the bounding prism/cylinder.
Cone-specific:
Vcone=31πr2h
SAcone=πr2+πrl
where h = perpendicular height, l = SLANT height.
h,r,l form a right triangle: h2+r2=l2 (Pythagoras).
Worked example. Cone with r=5, l=13.
- Find h: h2=169−25=144, h=12.
- V=31π×25×12=100π.
- Curved surface: π×5×13=65π.
- Total SA: 25π+65π=90π.
Worked qualitative. Why is cone volume 31 of cylinder volume (same r,h)?
- Cylinder: πr2h.
- Cone fits inside it.
- Calculus shows the cone is exactly 31.
- Geometrically: cones with the same base and apex tapered uniformly.
Edexcel tip. Pyramids/cones use PERPENDICULAR height for VOLUME, SLANT height for CURVED SURFACE. Don't confuse.