In triangle ABC, cos B = 3/5 and angle A is 90 degrees. What is cos (C + A)?
cos(C + A) = - 0.6
cos (C + A) = cosCcos A - sinC sinA
cos A = 0
sin C = 1
cos B = sin A
So
cos ( C + A) = 0 - (1)(3/5) = -3/5 = - .6
