5.4 Indefinite Integrals and the Net Change Theorem/1

$\int {\frac {x}{\sqrt {x^{2}+1}}}dx={\sqrt {x^{2}+1}}+c$

${\frac {d}{dx}}\left[(x^{2}+1)^{\frac {1}{2}}+c\right]={\frac {x}{\sqrt {x^{2}+1}}}$

let $a=x^{2}+1$ and $b=a^{1/2}$ then ${\frac {da}{dx}}=2x{\text{ and }}{\frac {db}{da}}={\frac {1}{2}}a^{-1/2}$

${\frac {da}{dx}}\cdot {\frac {db}{da}}=\left(2x\right)\left({\frac {1}{2}}a^{-1/2}\right)=xa^{-1/2}=x(x^{2}+1)^{-1/2}={\frac {x}{\sqrt {x^{2}+1}}}$