Câu 41: $C$
$\sin15^o-\cos15^o$
$=\sin15^o-\sin75^o$
$=2\cos\dfrac{15^o+75^o}{2}.\sin\dfrac{15^o-75^o}{2}$
$=-2\cos45^o.\sin30^o$
$=-2.\dfrac{1}{2}\cos45^o$
$=-\cos45^o$
$=-\sin45^o$
$=\sin(-45^o)$
Câu 42: $D$
$\sin4x.\cos2x-\sin3x.\cos x$
$=\dfrac{1}{2}(\sin6x+\sin 2x)-\dfrac{1}{2}(\sin4x+\sin 2x)$
$=\dfrac{1}{2}(\sin6x-\sin4x)$
$=\dfrac{1}{2}.2\cos5x.\sin x$
$=\cos5x.\sin x$
Câu 43: $-0,25$
$\cos\pi.\sin\dfrac{7\pi}{12}.\cos\dfrac{5\pi}{12}$
$=-\dfrac{1}{2}\Big(\sin\pi+\sin\dfrac{\pi}{6}\Big)$
$=-\dfrac{1}{2}\Big(0+\dfrac{1}{2}\Big)$
$=-\dfrac{1}{4}$
$=-0,25$