Answer:
D. 87
Explanation:
You want to know the measure of the angle opposite the longest side in a triangle with side lengths 13, 16, and 20 inches.
Angle relations
The angle x is opposite the side of length 20 inches in this triangle, which is the longest side. That tells you x is the largest angle.
The largest angle in any triangle is never less than 60°. This eliminates all answer choices except the last one:
x = 87
Law of Cosines
If you want to go to the trouble to solve the triangle, the law of cosines is helpful. For sides a, b, c and angle C, it tells you ...
c² = a² +b² -2ab·cos(C)
Solving for the angle, we have ...
C = arccos((a² +b² -c²)/(2ab))
C = arccos((13² +16² -20²)/(2·13·16)) = arccos(25/416) ≈ 86.55°
x ≈ 87