15° is the difference between the two special angles 45° and 30° We use the formula cos(A-B) = cos(A)cos(B) + sin(A)sin(B) where A = 45° and B = 30° cos(15°) = cos(45°-30°) = cos(45°)cos(30°) + sin(45°)sin(30°) = + = + = Edwin
