# Why arcsinh$(x)$ is the primitive of $(x^2+1)^{-1/2}$

I have a question: How to calculate the following primitive of $g(x)$. $I=\int g(x)\text{d}x=\int\dfrac{\text{d}x}{\sqrt{x^2+1}}$.

I know that it is equal to the inverse of the sinus hyperbolic function. I want to know the steps to get the function $\sinh^{-1}x$. How to get this result?

Also, I saw in Wikipedia that $\sinh^{-1}x=\log(x+\sqrt{x^2+1})$ but when I calculate $(\sinh^{-1}x)^{\prime}=(\log(x+\sqrt{x^2+1}))^{\prime}$ I do not get $g(x)$. Any explanation please?

You know that the derivative of the inverse function $f^{-1}$ is:

$$\left(f^{-1}\right)'(y)=\frac1{f'(f^{-1}(y))}$$

and recall that $$\cosh^2 y-\sinh^2y=1$$ hence using the last equality we find $$(\sinh^{-1})'(y)=\frac1{\cosh(\sinh^{-1}(y))}=\frac1{\sqrt{1+y^2}}$$

• Thank you very much. Why when I use the formula with logarithm I cannot get the right answer? – zighalo Apr 4 '14 at 16:24
• If you derivate the $\log$ expression you find $\frac1{\sqrt{x^2+1}}$ so what you can deduce? – user63181 Apr 4 '14 at 16:27
• I deduce that I have the right answer. But when I derivate the $\log$ expression I get: $\dfrac{1+(1/2)2x(x^2+1)^{-1/2}}{x+\sqrt{x^2+1}}$. I tried to simplfy it to get $\dfrac{1}{\sqrt{x^2+1}}$ but I cannot. Where is my wrong move? – zighalo Apr 4 '14 at 16:31
• Your calculus is true just multiply numerator and denominator by $(x^2+1)^{1/2}$ and simplify the numerator. – user63181 Apr 4 '14 at 16:34
• Ohh I got it. Thank you very mcuh for your help. – zighalo Apr 4 '14 at 16:46

To emphasize how to get there: substitute $$x = \sinh t, \; \; dx = \cosh t \, dt$$ and $$\sqrt {1 + x^2} = \sqrt {1 + \sinh^2 t} = \sqrt {\cosh^2 t} = \cosh t.$$ The substituted integral is now $$\int \frac{1}{\cosh t} \; \cosh t \; dt = \int 1 \, dt = t + C$$ But $x = \sinh t$ and $t = \operatorname{argsinh} x,$ so the integral really is $$\operatorname{argsinh} x + C$$