Chứng minh rằng (sin^2 a + cos^2 a)^2 – (sin^2 a – cos^2 a)^2/ sin a. cos a = 4 giải giúp em với ạ

$\dfrac{(\sin a + \cos a)^2 – (\sin a – \cos a)^2}{\sin a\cos a}$

$= \dfrac{\sin^2a + \cos^2a + 2\sin a\cos a – (\sin^2 + \cos^2x – 2\sin x\cos x)}{\sin a\cos a}$

$=\dfrac{1 + 2\sin a\cos a – (1 – 2\sin a\cos a)}{\sin a\cos a}$

$= \dfrac{4\sin a\cos a}{\sin a\cos a}$

$= 4$