Notice the little right triangle (5). The shaded angle is A, because the line on its top side is parallel to the base line. Similar right triangles with an angle A show that the top angle, marked A, also equals the original A. The top part of the opposite (6), over the longest of that shaded triangle, is cos A. The opposite over the main hypotenuse (7) is sin B. Since the side marked "opposite" (7) is in both the numerator and denominator when cos A and sin B are multiplied together, cos A sin B is the top part of the original opposite — for (A + B) — divided by the main hypotenuse (8).. As the examples showed, sometimes we need angles other than 0, 30, 45, 60, and 90 degrees. In this chapter you need to learn two things: 1. Sin(A + B) is not equal to sin A + sin B.. If A and A+B are acute angles so that sin(A+B)=24/25 and tan A=3/4 then find the value of cos B?.