Find the derivative of `y=sqrt(x^2+1)`.

Explanation

Here `y=f(g(x))`, where `f(u)=g(x)` and `g(x)=x^2+1`. Since the derivates of `f` and `g` are
`f^'(u)=(1)/(2sqrtu)` and `g^'(x)=2x`,
the chain rule gives
`(dy)/(dx)=(d)/(dx)f(g(x))`
`=f^'(g(x)).g^'(x)`
`=(1)/(2sqrt(g(x)))*g^'(x)=(1)/(2sqrt(x^2+1))*(2x)=(x)/(sqrt(x^2+1))`