Question 702689
If sin x=1/√5,
and cos x =sqrt(1-sin^2x)=sqrt(1-1/5)=+/- 2/√5 is less than 0
so cos x =- 2/√5
 cos 2x=cos^2x-sin^2x=[- 2/√5]^2-[1/√5]^2=4/5-1/5=3/5
ans