

Chứng minh đẳng thức:
[tex]\left ( sin^{4}\alpha + cos^{4}\alpha -1 \right )\left ( tan^{2}\alpha + cot^{2}\alpha +2 \right ) = 2[/tex]
[tex]\left ( sin^{4}\alpha + cos^{4}\alpha -1 \right )\left ( tan^{2}\alpha + cot^{2}\alpha +2 \right ) = 2[/tex]