Rút gọn biểu thức A= sin^4x-cos^4x + sin2x /tan2x-1

$A=\dfrac{\sin^4x-\cos^4x+\sin2x}{\tan2x-1}$

$=\dfrac{\sin^2x-\cos^2x+\sin2x}{\tan2x-1}$

$=\dfrac{\sin^2x-\cos^2x+\sin2x}{\frac{\sin2x-\cos2x}{\cos2x}}$

$=\dfrac{\cos2x(\sin^2x-\cos^2x+\sin2x)}{\sin2x-\cos2x}$

$=\dfrac{\cos2x(-\cos 2x+\sin2x)}{-\cos2x+\sin2x}$

$=\cos 2x$