Two ships, one sailing at 30 km/hr and the other at 45 km/hr, left port at the same time. Three hours later they were 120 km apart. If you had to find the angle between their courses an equation that could be used to solve this problem is:

Answer 1

Do you not have a triangle with sides
90, 135 and 120 ?

So use the cosine law starting with
120^2 = 90^2 + 135^2 - 2(90)(135cos A , where A is the angle opposite the 120