khong biết làm huhuhu

Đáp án: $D$

Giải thích các bước giải:

Ta có:

$M=\cos10^o\cos20^o\cos40^o\cos80^o$

$\to M=\cos10^o\cos20^o\cos40^o\sin(90^o-80^o)$

$\to M=\cos10^o\cos20^o\cos40^o\sin(10^o)$

$\to M=\sin(10^o)\cos10^o\cos20^o\cos40^o$

$\to M=\dfrac12\cdot 2\sin(10^o)\cos10^o\cos20^o\cos40^o$

$\to M=\dfrac12\cdot \sin20^o\cos20^o\cos40^o$

$\to M=\dfrac14\cdot 2\sin20^o\cos20^o\cos40^o$

$\to M=\dfrac14\cdot\sin40^o\cos40^o$

$\to M=\dfrac18\cdot2\sin40^o\cos40^o$

$\to M=\dfrac18\cdot\sin80^o$

$\to M=\dfrac18\cdot\cos(90^o-80^o)$

$\to M=\dfrac18\cdot\cos10^o$

$\to D$