Chapter 13, Trigonometric Ratios and Functions - 13.6 Apply the Law of Cosines - 13.6 Exercises - Skill Practice - Page 892: 33

B

By Heron's Formula, we know: $$A=\sqrt{s(s-a)(s-b)(s-c)}$$ Where: $$s=\frac{a+b+c}{2}$$ Thus, we find: $$ s=\frac{21+16+13}{2}=25 \\ A=\sqrt{25\left(25-21\right)\left(25-16\right)\left(25-13\right)} \\ A\approx 104$$