+129852 cos (A -90) + 1 = 2(cosA)
cosA cos90 + sinAsin90 + 1 = 2cosA
sinA + 1 = 2 cos A square both sides
sin^2 A + 2sin A + 1 = 4cos^2 A
sin^2A + 2sin A + 1 = 4 ( 1 - sin^2A)
sin^2A + 2sin A + 1 = 4 - 4sin^2 rearrange as
5sin^2A + 2sinA - 3 = 0
(5sin A - 3) ( sin A + 1) = 0
5sin A - 3 = 0 or sin A = -1 A = 3pi/2 cos (3pi/2) = 0
sin A = 3/5
so cos A =4/5 or cos A = 0
Proof
sin A + 1 = 2cosA sin A + 1 = 2cosA
3/5 + 1 = 2(4/5) -1 + 1 = 2(0)
3/5 + 5/5 = 8/5 0 = 0
8/5 = 8/5
