1)$tan^2 x.cos^2 2x.cot 3x=tan^2 x-cot^2 2x+cot 3x$$\Leftrightarrow cot 3x(cot^22x.tan^2 x-1)=tan^2 x-cot^2 2x$
$\Leftrightarrow cot 3x=\frac{tan^2 x-cot^2 2x}{cot^2 2x.tan^2 x-1}$
Thay $cot 2x =\frac{1}{tan 2x}$ và rút gọn ta đc:$cot 3x= \frac{tan^2x.tan^2 2x-1}{tan^2 x-tan^2 2x}$
$\Rightarrow cot 3x= \frac{(1-tan x.tan 2x)(1+tan x.tan 2x)}{(tan 2x-tan x)(tan 2x+tan x)}=cot 3x.cot x$