Areacosh/ln x + sqrt (x^2-1)/Aufgabe/Lösung

Es ist

{}{\begin{aligned}\cosh {\left(\ln {\left(x+{\sqrt {x^{2}-1}}\right)}\right)}&={\frac {1}{2}}{\left(e^{\ln {\left(x+{\sqrt {x^{2}-1}}\right)}}+e^{-\ln {\left(x+{\sqrt {x^{2}-1}}\right)}}\right)}\\&={\frac {1}{2}}{\left({\left(x+{\sqrt {x^{2}-1}}\right)}+{\frac {1}{x+{\sqrt {x^{2}-1}}}}\right)}\\&={\frac {1}{2}}{\frac {{\left(x+{\sqrt {x^{2}-1}}\right)}^{2}+1}{x+{\sqrt {x^{2}-1}}}}\\&={\frac {1}{2}}{\frac {x^{2}+2x{\sqrt {x^{2}-1}}+x^{2}-1+1}{x+{\sqrt {x^{2}-1}}}}\\&={\frac {x^{2}+x{\sqrt {x^{2}-1}}}{x+{\sqrt {x^{2}-1}}}}\\&=x.\end{aligned}}

Wir wenden die Umkehrfunktion ${}\,\operatorname {arcosh} \,x\,$ auf diese Gleichung an und erhalten

{}\ln {\left(x+{\sqrt {x^{2}-1}}\right)}=\,\operatorname {arcosh} \,x\,\,.