tan^2a=2tan^2b+1sin^2a/cos^2a=2*sin^2b/cos^2b+2-1sin^2a/cos^2a+1=2*sin^2b/cos^2b+21/cos^2a=2/cos^2bcos^2b=2*cos^2a所以1-sin^2b = 2(1-sin^2)=2-2sin^2a整理得sin^b=2sin^2a-1