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° 129.9° 136.2° 50.1°.

x = 129.9 degrees to the nearest tenth.

Step-by-step explanation:

The largest angle will be opposite the longest side (20 in). So by the Cosine Rule:

20^2 = 9^2 + 13^2 - 2*9*13 cos x where x is the largest angle.

cos x = (20^2-0.640 - 9^2 - 13^2) / (-2*9*13)

cos x = - 0.641025641

x = 129.87 degrees.