Sina+cosa=0.8 sinacosa=?

Question

Sina+cosa=0.8 sinacosa=?

Answer 1

0.64 = (sin a + cos a)^2 = (sin^2 a + cos^2 a) + 2 sin a cos a = 1 + 2 sin a cos a
sin a cos a = (0.64 - 1)/2 = -0.18