1.$f\left( x\right) =\dfrac {\tan x}{\cot x}+\dfrac {\cot x}{\tan x}$
2.$f\left( x\right) =\dfrac {\dfrac {\sin x}{\cos x}}{\dfrac {\cos x}{\sin x}}+\dfrac {\dfrac {\cos x}{\sin x}}{\dfrac {\sin x}{\cos x}}$
3.$f\left( x\right) =\dfrac {\sin ^{2}x}{\cos ^{2}x}+\dfrac {\cos ^{2}x}{\sin ^{2}x}$
4.$f\left( x\right) =\tan ^{2}x+\cot ^{2}x$
5.
- $\left( \tan ^{2}x\right)' =2.\tan x.\dfrac {d}{dx}\left( \tan x\right)=2.\tan x.sec^2x$
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$\left( \cot ^{2}x\right)' =2.\cot x.\dfrac{d}{dx}\left(\cot x\right)=2.\cot x.csc^2x$
6.$f(x)'=2\tan x.\sec^2 x+2\cot x.\csc^2 x$