b. ĐKXĐ: \(x\ne\dfrac{\pi}{2}+k\pi\)
\(y=\dfrac{sinx-cosx}{cosx}\)
\(\Rightarrow y'=\dfrac{\left(cosx+sinx\right)cosx+sinx\left(sinx-cosx\right)}{cos^2x}\)
\(\Leftrightarrow y'=\dfrac{sinx.cosx+cos^2x+sin^2x-cosx.sinx}{cos^2x}\)\(=\dfrac{1}{cos^2x}\)