A triangle has sides A, B, and C. Sides A and B are of lengths #2# and #5#, respectively, and the angle between A and B is #(5pi)/8 #. What is the length of side C?
Use the cosine law in this situation. The cosine law states the following for ANY triangle.
With Y= angle between a and b.
If the angle is nice and the cosine gives a value displayed in pi, the senses is much nicer. For this calculation one cannot simplify it.