sin(a-2派)=sina,sin(派+a)=-sina,tan(派—a)=-tana,cot（a—派）=cota
2cos^2(-a)= ,tana*cota=1,sin^2a+cos^2a=1
原式=1-sin^2a+1-cos^2a-cos^2a=1+1-1-cos^2a=1-cos^2a=sin^2a