Question

Given that Sin A = 4/5, 0 < A < π/2 and Cos B = -12/13, π< B < 3π/2, find Cos 2A

Answer 1

Cos 2A= 1-2 sin^2 A
=1-2 (4/5)^2
=1-2(16/25)
=1-32/25
=-7/25.
I don't know why there is a need of Cos B.

Answer 2

But Cos is positive in 0<A<90.So answer is 7/25

Answer 3

thank you so much

Answer 4

no problem...