1) $(1-sinx+cosx)^2=2(1-sinx)(1+cosx)$$<=>1+sin^2+cosx^2-2sinx+2cosx-2sinxcosx=2-2sinx+2cosx-2sinxcosx$(luôn đúng)
2)$ sin^2x(1+cot^2x)=3cos^2x(1+tan^2x)-2$
$<=>sinx^2.\frac{1}{sinx^2}=3cosx^2.\frac{1}{cosx^2}-2$
$<=>-2=-2$(luôn đúng)
3) $cos^4x-sin^4x= cos^2x(1-tanx)(1+tanx)$
$<=>(cosx^2)^2-(sinx^2)^2=cosx^2(1-tanx^2)$
$<=>cosx^2-sinx^2=cosx^2-sinx^2$(luôn đúng)