I assumed $\sin^{-1}(x)=A$, $\sin^{-1}(y)=B$ and $\sin^{-1}(z)=C$.
Then it went the following way
$ A + B + C = \pi$
$A + B = \pi - C$
$\sin(A+B)= \sin(\pi-C)$
$\sin A \cos B + \cos A \sin B = sin C$
$x\sqrt{1-y^2}+y\sqrt{1-x^2} = \sin(\sin^{-1}(z))$
Am i right in doing so or heading wrong?
Also, the formula you've given is not true, thus it cannot be proven. Please check if it's correctly written.
– Adam Latosiński Apr 30 '19 at 07:04