Hey guys

how do I prove that the angle between the asymptotes of a rectangular hyperbola is 90 degrees

I derived the formula from a previous step in the question to find the angle between $\displaystyle \frac{x^{2}}{a^{2}}-\frac{y^{2}}{b^{2}}=1$ which was $\displaystyle \theta=\tan^{-1} (\frac{2ab}{a^{2}-b^{2}})$

so now that in a rectangular hyperbola $\displaystyle x^{2}-y^{2}=a^{2}$..and now that $\displaystyle a^2 =b^2$, how do i prove that the angle is $\displaystyle 90^\circ$

Thanks