(1)左边=(sina)^2/(cosa)^2-(sina)^2=(sina)^2*[1/(cosa)^2-1]==(sina)^2*[(1-(cosa)^2)/(cosa)^2]=右边
(2)左边=(cosβ)^2-2cosβ+1+(sinβ)^2=右边
(3)左边=[(sinx)^2+(cosx)^2]^2-2(sinx)^2*(cosx)^2=右边
(4)应该是tanα=3吧?若是,则①=(4tanα-2)/5+3*3=11；②sinα/cosα=3,(sinα)^2=3(cosα)^2,又,(sinα)^2+(cosα)^2=1,故(cosα)^2=1/4,则原式=(sinα/cosα)*(cosα)^2=3/4
③原式=1+2sinαcosα=1+6/4=5/2
(5)cosα=1/4,sinα=±√（1-1/4)=±√3/2,tanα=(±√3/2)/1/4)=±2√3
(6)π/2