Chứng minh rằng: $(tanx+\frac{cosx}{1+sinx})(cotx+\frac{sinx}{1+cosx})(\frac{cosx-cos3x}{4sinx})=1$.
Trong đó: $sinx.cosx(1+cosx)(1+sinx) \neq 0$
$LHS=(\frac{sinx}{cosx}+\frac{cosx}{1+sinx})(\frac{cosx}{sinx}+\frac{sinx}{1+cosx})(\frac{4cosx(1-cosx^{2})}{4sinx})=\frac{1}{sinx}.\frac{1}{cosx}.\frac{4cosx.sinx^{2}}{4sinx}=1$