Question

find the exact value of : cos(2 arctan(4/3))

Answer 1

cos(2 arctan(4/3)) = cos(2 arccos(3/5)) = cos^2(arccos(3/5)) - sin^2(arccos(3/5)) = cos^2(arccos(3/5)) - sin^2(arcsin(4/5)) = (3/5)^2 - (4/5)^2 = 9/25 - 16/25 = -7/25.