cos(2tan^-1(15/8))

Answer 1

If θ = tan^-1(15/8)
sinθ = 15/17
cosθ = 8/17

now use the double angle formula

cos2θ = 2cos^2 θ - 1 = 2(64/289)-1 = -161/289

cos2θ = 1 - 2sin^2 θ = 1-450/289 = -161/289