The three sides of a triangular plot measure 13 m, 14 m and 15 m. What is its area in square metres?

Use Heron'$s$ formula with $s=(13+14+15)/2=21$: area = √$(21\times 8\times 7\times 6)$ = √7056 = 84 sq m. The trap $91=(1/2)\times 13\times 14$ treats two sides as a base and a perpendicular height, but no angle in a 13-14-15 triangle is a right angle; 42 halves the Heron result once more out of habit; 21 is the semi-perimeter, a length rather than an area.