Find cos2A when tanA = 1/3 and π<A<3π/2.
A) 3/4
B) 5/4
C) 4/5
D) 3/5
cos2A = cos^2A - sin^2A
sin A = 1/√10 cosA = 3/ √10 .....so....
cos2A= (3/ √10)^2 - (1/ √10)^2 = 9/10 - 1/10 = 8/10 = 4/5
