(cos2a-sin^2 a)/(2sin^2a-cos^2a)=(cos^2a-sin^2a-sin^2a)/(2sin^2a-cos^2a)=
=(1-3sin^2a)/(3sin^2a-1)=-1
(cos2a-sin^2 a)/(2sin^2a-cos^2a)=(cos^2a-sin^2a-sin^2a)/(2sin^2a-cos^2a)=
=(1-3sin^2a)/(3sin^2a-1)=-1