a. Given that is acute and tan = 3/4,prove that 3 sin ( + )

a. Given that α is acute and tan α = 3/4,prove that 3 sin (θ + α) + 4 cos (θ + α) ≡ 5 cos θ

b. Given that sin x = 0.6 and cos x = −0.8, evaluate cos (x  270°) and cos (x + 540°).