\(\begin{array}{l}a)\,\,dy = d\left( {{{\tan }^2}x} \right)\\ \Rightarrow dy = \left( {{{\tan }^2}x} \right)'dx\\\,\,\,\,\,\,dy = 2\tan x.\dfrac{1}{{{{\cos }^2}x}}dx\\\,\,\,\,\,dy = \dfrac{{2\sin x}}{{{{\cos }^3}x}}dx\\b)\,\,dy = d\left( {\dfrac{{\cos x}}{{1 - {x^2}}}} \right)\\ \Rightarrow dy = \left( {\dfrac{{\cos x}}{{1 - {x^2}}}} \right)'dx\\\,\,\,\,\,dy = \dfrac{{ - \sin x\left( {1 - {x^2}} \right) + 2x\cos x}}{{{{\left( {1 - {x^2}} \right)}^2}}}dx\end{array}\)
