Cho tui hỏi tí, có công thức hạ bậc của sin³x hay sin mũ bốn x hay không nhỉ :(( Nếu có thì cho tui xin với :(

$\sin^3x=\sin x\sin^2x$

$=\sin x.\dfrac{1-\cos2x}{2}$

$=\dfrac{\sin x-\sin x\cos2x}{2}$

$=\dfrac{1}{2}\sin x-\dfrac{1}{2}.\dfrac{1}{2}(\sin3x-\sin x)$

$=\dfrac{-1}{4}\sin3x+\dfrac{3}{4}\sin x$

$\sin^4x=(\sin^2x)^2$

$=\dfrac{(1-\cos2x)^2}{4}$

$=\dfrac{1-2\cos2x+\cos^22x}{4}$

$=\dfrac{1}{4}-\dfrac{1}{2}\cos2x+\dfrac{1}{4}\cos^22x$

$=\dfrac{1}{4}-\dfrac{1}{2}\cos2x+\dfrac{1}{4}.\Big(\dfrac{1}{2}+\dfrac{1}{2}\cos4x \Big)$

$=\dfrac{1}{8}\cos4x-\dfrac{1}{2}\cos2x+\dfrac{3}{8}$