Đáp án:
$\dfrac{\sin\dfrac{16x}{5}}{16\sin\dfrac{x}{5}}$
Giải thích các bước giải:
$\quad \cos\dfrac x5\cos\dfrac{2x}{5}\cos\dfrac{4x}{5}\cos\dfrac{8x}{5}$
$= \dfrac{2\sin\dfrac x5\cos\dfrac x5\cos\dfrac{2x}{5}\cos\dfrac{4x}{5}\cos\dfrac{8x}{5}}{2\sin\dfrac x5}$
$= \dfrac{\sin\dfrac{2x}{5}\cos\dfrac{2x}{5}\cos\dfrac{4x}{5}\cos\dfrac{8x}{5}}{2\sin\dfrac x5}$
$= \dfrac{2\sin\dfrac{2x}{5}\cos\dfrac{2x}{5}\cos\dfrac{4x}{5}\cos\dfrac{8x}{5}}{4\sin\dfrac x5}$
$= \dfrac{\sin\dfrac{4x}{5}\cos\dfrac{4x}{5}\cos\dfrac{8x}{5}}{4\sin\dfrac x5}$
$=\dfrac{2\sin\dfrac{4x}{5}\cos\dfrac{4x}{5}\cos\dfrac{8x}{5}}{8\sin\dfrac x5}$
$= \dfrac{\sin\dfrac{8x}{5}\cos\dfrac{8x}{5}}{8\sin\dfrac x5}$
$= \dfrac{2\sin\dfrac{8x}{5}\cos\dfrac{8x}{5}}{16\sin\dfrac x5}$
$=\dfrac{\sin\dfrac{16x}{5}}{16\sin\dfrac{x}{5}}$
