Primitive function/Square root of x^2-1/Example

We determine a primitive function for ${}{\sqrt {x^{2}-1}}$, using the hyperbolic functions ${}\sinh t$ and ${}\cosh t$, for which the relation ${}\cosh ^{2}t-\sinh ^{2}t=1$ holds. The substitution

$x=\cosh t{\text{ with }}dx=\sinh tdt$

yields

${}\int _{a}^{b}{\sqrt {x^{2}-1}}\,dx=\int _{\,\operatorname {arcosh} \,a\,}^{\,\operatorname {arcosh} \,b\,}{\sqrt {\cosh ^{2}t-1}}\cdot \sinh t\,dt=\int _{\,\operatorname {arcosh} \,a\,}^{\,\operatorname {arcosh} \,b\,}\sinh ^{2}t\,dt\,.$

A primitive function of the hyperbolic sine squared follows from

${}\sinh ^{2}t={\left({\frac {1}{2}}{\left(e^{t}-e^{-t}\right)}\right)}^{2}={\frac {1}{4}}{\left(e^{2t}+e^{-2t}-2\right)}\,.$

Therefore,

${}\int _{}^{}\sinh ^{2}u\,dt={\frac {1}{4}}{\left({\frac {1}{2}}e^{2u}-{\frac {1}{2}}e^{-2u}-2u\right)}={\frac {1}{4}}\sinh 2u-{\frac {1}{2}}u\,,$

and hence

${}\int _{}^{}{\sqrt {x^{2}-1}}\,dx={\frac {1}{4}}\sinh(2\,\operatorname {arcosh} \,x\,)-{\frac {1}{2}}\,\operatorname {arcosh} \,x\,\,.$

Due to the addition theorem for hyperbolic sine, we have ${}\sinh 2u=2\sinh u\cosh u$, and therefore this primitive function can also be written as

${}{\begin{aligned}{\frac {1}{2}}{\left(\sinh {\left(\,\operatorname {arcosh} \,x\,\right)}\cosh {\left(\,\operatorname {arcosh} \,x\,\right)}-\,\operatorname {arcosh} \,x\,\right)}&={\frac {1}{2}}{\left({\sqrt {\cosh {\left(\,\operatorname {arcosh} \,x\,\right)}^{2}-1}}\cdot x-\,\operatorname {arcosh} \,x\,\right)}\\&={\frac {1}{2}}{\left({\sqrt {x^{2}-1}}\cdot x-\,\operatorname {arcosh} \,x\,\right)}.\end{aligned}}$