We use the 30-60 and 45-45 triangles again. In this case, for the cosine of the difference of two angles, we have:

`{:(cos(beta-alpha),=cos\ beta\ cos\ alpha+sin\ beta\ sin\ alpha),(,=((-12)/13)3/5+(5/13)4/5),(,=(-36+20)/65),(,=(-16)/65):}`