Giải thích các bước giải:
b.$\cos^3a.\sin a-\sin^3a.\cos a=\sin a\cos a(\cos^2a-\sin^2a)=\dfrac12\sin 2a.\cos 2a=\dfrac14\sin 4a$
e.$E=\dfrac{\sin3x+\sin x}{\cos3x+\cos x}$
$\to E=\dfrac{3\sin x-4\sin^3x+\sin x}{4\cos^3x-3\cos x+\cos x}$
$\to E=\dfrac{4\sin x-4\sin^3x}{4\cos^3x-2\cos x}$
$\to E=\dfrac{2\sin x(1-\sin^2x)}{\cos x(2\cos^2x-1)}$
$\to E=\dfrac{2\sin x\cos^2x}{\cos x\cos2x}$
$\to E=\dfrac{2\sin x\cos x}{\cos2x}$
$\to E=\dfrac{\sin2x}{\cos2x}$
$\to E=\tan2x$
