If ​, find x.

if you square it,

 x+ $\sqrt{x+\sqrt{x+...}}$=81

so x=72

Since the square root goes on for infinity:

$\sqrt{x+\sqrt{x+\sqrt{x+\sqrt{x...}}}}=\sqrt{x+\sqrt{x+\sqrt{x...}}}$

Let us set $\sqrt{x+\sqrt{x+\sqrt{x+\sqrt{x...}}}}=y$

This means: $y=9$

This also means: $\sqrt{x+y}=9$

Substitute $y$: $\sqrt{x+9}=9$

Square both sides: $x+9=81$

$\boxed{x=72}$