going through practice problems for a test

Am I doing this correctly?

Find the differential:

$\displaystyle y=\sqrt{9-x^2}=(9-x^2)^\frac{1}{2}$

via power rule: $\displaystyle y'=\frac{1}{2}(9-x^2)^\frac{-1}{2}$

and so,

$\displaystyle dy=\frac{1}{\sqrt{4.5-.5x^2}}*dx$

thank you in advance