## Elementary Geometry for College Students (6th Edition)

84 $cm^{2}$
Given sides of a triangle 10 cm, 17 cm, 21 cm Using Heron's formula if three sides of a triangle have length a,b,c then area A of a triangle is A = $\sqrt s(s-a)(s-b)(s-c)$ where s = $\frac{a+b+c}{2}$ Lets take a=10 b=17 and c=21 s = $\frac{10+17+21}{2}$ = 24 cm A = $\sqrt 24(24-10)(24-17)(24-21)$ =$\sqrt 24*14*7*3$ =84 $cm^{2}$