$\Leftrightarrow \int\limits_{}^{}\frac{\frac{dx}{cos^2x}}{(tanx +1)^2} = \int\limits_{}^{}\frac{(tanx +1)' dx}{(tanx +1)^2} = \int\limits_{}^{} \frac{d(tanx+1)}{(tanx +1)^2}= -\frac{1}{tanx +1}...$ta có d(fx) = f'x dx
$\Leftrightarrow \int\limits_{}^{}\frac{\frac{dx}{cos^2x}}{(tanx +1)^2} = \int\limits_{}^{}\frac{(tanx +1)' dx}{(tanx +1)^2} = \int\limits_{}^{} \frac{d(tanx+1)}{(tanx +1)^2}= -\frac{1}{conx +1}...$ta có d(fx) = f'x dx
$\Leftrightarrow \int\limits_{}^{}\frac{\frac{dx}{cos^2x}}{(tanx +1)^2} = \int\limits_{}^{}\frac{(tanx +1)' dx}{(tanx +1)^2} = \int\limits_{}^{} \frac{d(tanx+1)}{(tanx +1)^2}= -\frac{1}{
tanx +1}...$ta có d(fx) = f'x dx