Tính:
a. $sin\frac{\pi}{8}cos\frac{\pi}{8}cos\frac{\pi}{4}$
b. $\frac{tan15^{\circ}}{1-tan^215^o}$
c. $\frac{1-tan^2\frac{\pi}{8}}{tan\frac{\pi}{8}}$
d. $cos165^o$
e. $16sin10^osin30^osin50^osin70^osin90^o$
f. $sin6^osin42^osin66^osin78^o$
g. $cos\frac{\pi}{7}cos\frac{4\pi}{7}cos\frac{5\pi}{7}$
h. $cos\frac{\pi}{65}cos\frac{2\pi}{65}cos\frac{4\pi}{65}cos\frac{8\pi}{65}cos\frac{16\pi}{65}cos\frac{32\pi}{65}$