This is a case for solving a triangle given the value of its three sides. SInce no angle value is provided, the most convenient way to solve it is using the law of cosines which states:
[tex]c^2=a^2+b^2-2ab\cdot\cos (C)[/tex]So in the expression above it is easy to solve for the cosine value of the angle in question.
Since they don't give us which angle is the larger acute angle in the problem, we need to figure it out based on the value of the sides. Notice that there are two sides pretty large (12 and 13 in length) and one which is very small in comparison (5)
