Question

A triangle has side lengths of 9 in, 13 in, and 20 in. What is the measurement of this triangle’s largest angle?
29.9°
50.1°
129.9°
136.2°

Answer 1

When we know all 3 sides, we use the Law of Cosines:

The largest side is 20 so C is the largest angle

cos(C) = (a^2 + b^2 -c^2) / (2*a*b)
cos (C) = (9^2 + 13^2 -20^2)/ =2*9*13
cos (C) = (81 + 169 -400) / 234
cos (C) = -150 / 234
cos (C) = -0.641025641
The arc cosine of -0.641025641 is
129.87 Degrees

Answer 2

that is correct