Evaluate: cos(2arctan(1/3))

i used the double angle identity cos(2x) = cos^2x-sin^2x
cos(2x)= (3/sqrt(10))^2- (1/sqrt(10))^2 = 2/25
2/25 was my answer to this problem but the correct answer was 4/5
how is the correct answer 4/5? ? i thought i did everything right