1, CM : a) $\frac{sin^3x}{1+cosx}=sinx + \frac{1}{2}sin2x$
b) $8sin^4x=3-4cos2x+cos4x$
c) $\frac{sinx+cosx}{cos^3x}=1+tanx+tan^2x+tan^3x$
2. Rút gọn
A= $\sqrt{2+\sqrt{2+2cosx}} với 0\leq x\leq \pi $
3. CMR k phụ thuộc vào x:
a) $cos^4x(3-2cos^2x)+sin^4x(3-2sin^2x)$
b) $\frac{tanx}{tan^2x-1}.\frac{cot^2x-1}{cotx}$
c) $cos^2x+cos^2(\frac{2\pi }{3}+x)+cos^2(\frac{2\pi }{3}-x)$
d) $(sin^4+cos^4-1)(tan^2x+cot^2x+2)$