a) [imath]y'= (\dfrac{x+1}{x^2})'.\dfrac{1}{\cos^2{\dfrac{x+1}{x^2}}}[/imath]
[imath]y'=\dfrac{-x-2}{x^3}.\dfrac{1}{\cos^2{\dfrac{x+1}{x^2}}}[/imath]
b) [imath]\dfrac{1-tan^2x}{1+tan^2x}=cos^2x[(1+tan^2x)-2tan^2x][/imath]
[imath]=1-2sin^2x=cos2x[/imath]
Khi đó [imath]y'= [(cos2x)^{20}]'=-20.cos^{19}2x.2.\sin{2x}[/imath]
Bạn tham khảo nhé. Chúc bạn học tốt!!!