I need help with double angle formulas cosx1026 and lies in

I need help with double angle formulas.

cosx=10/26 and lies in Q4.

Find the exact values of a) sin2x b) cos2x and c)tan2x

Solution

Cos x=10/26=5/13

sin x =sqrt(1-cos²x) =sqrt(1-25/169)=- 12/13

cos (2x)=2cos²x-1 = 2(25/169)-1 = -119/169

sin(2x)=2 sin x cos x= -120/169

tan 2x=sin2x/cos 2x= 119/120