rút gọn bt cosa.cos2a.cos4a.cos8a.cos16a

Đặt $P=\cos a.\cos 2a.\cos 4a.\cos8a.\cos16a$

$P\sin a=\sin a.\cos a.\cos2a.\cos4a\cos8a.\cos16a$

$=\dfrac{1}{2}\sin2a.\cos2a.\cos4a.\cos8a.\cos16a$

$=\dfrac{1}{4}\sin4a.\cos4a.\cos8a.\cos16a$

$=\dfrac{1}{8}\sin8a.\cos8a.\cos16a$

$=\dfrac{1}{16}\sin16a.\cos16a$

$=\dfrac{1}{32}\sin32a$

Vậy: $P=\dfrac{\sin32a}{32\sin a}$