The Cone: Slant First

A cone given by its radius and its height needs one more step before its surface area: the slant height, from the right triangle inside.

This cone has radius 5 and height 12, so ${s}^{2} = {5}^{2}+{12}^{2} = 169$ and s = 13. Then $\pi\cdot {5}^{2}+\pi\cdot 5\cdot 13 = 25\cdot \pi+65\cdot \pi = 90\cdot \pi$.

Two steps on the board: tap the slant height and pick Slant height, then tap the cone and pick Surface area of a cone.