Giải thích các bước giải:
a.$\cos x-\sin 3x=0$
$\to \cos x=\sin 3x$
$\to \cos x=\cos(3x-\dfrac{\pi}{2})$
$\to x=3x-\dfrac{\pi}2+k2\pi\to x=\dfrac{\pi}4-k\pi$
Hoặc $x=-3x+\dfrac{\pi}2+k2\pi\to x=\dfrac{\pi}{8}+\dfrac{k\pi}2$
b.$\sin x+\cos x=\sqrt{2}\sin 2x$
$\to \dfrac{1}{\sqrt{2}}\sin x+\dfrac{1}{\sqrt{2}}\cos x=\sin 2x$
$\to \sin(x+\dfrac{\pi}4)=\sin 2x$
$\to x+\dfrac{\pi}4=2x+k2\pi\to x=\dfrac{\pi}4-k2\pi$
Hoặc $x+\dfrac{\pi}4=\pi-2x+k2\pi\to x=\dfrac{\pi}4+\dfrac{k2\pi}3$
