# Math Help - Can somebody prove - Hyperbolic functions

1. ## Can somebody prove - Hyperbolic functions

Could somebody please prove these identities and explain a little bit ? I just can't figure them out.

sinh^-1x = ln(x + sq.rt of(x^2 + 1))

cosh^-1x = ln(x + sq.rt of (x^2 + 1))

tanh^-1x = (1/2)ln ( (1+x)/(1 - x) )

2. Originally Posted by yakkow
Could somebody please prove these identities and explain a little bit ? I just can't figure them out.

sinh^-1x = ln(x + sq.rt of(x^2 + 1))
I prove just one, you prove the rest.
Since,
$y=\sinh x$ is a bijective function it has an inverse.
Thus you interchange the x and y in,
$y=\frac{e^x-e^{-x}}{2}$
To get,
$x=\frac{e^y-e^{-y}}{2}$
Thus,
$2x=e^y-e^{-y}$
Thus,
$e^{2y}-1=2xe^{y}$
Thus,
$e^{2y}-2xe^y-1=0$
Allow, $t=e^y$ thus,
$t^2-2xt-1=0$
Thus,
$t=\frac{2x\pm\sqrt{4x^2+4}}{2}$
Thus,
$t=\frac{2x\pm 2\sqrt{x^2+1}}{2}$
Which is,
$t=x\pm \sqrt{x^2+1}$
Since, $t>0$ we have,
$t=x+\sqrt{x^2+1}$
Thus,
$e^y=x+\sqrt{x^2+1}$
Thus,
$\sinh^{-1}x=\ln (x+\sqrt{x^2+1})$