16. Find the second derivative of the function. f(x)=\sqrt{9-x^{2}} f^{\prime \prime}(x)=\frac{9 x}{\left(9-x^{2}\right)^{3 / 2}} f^{\prime \prime}(x)=\frac{x^{2}+9}{\left(9-x^{2}\right)^{3 / 2}} f^{\prime \prime}(x)=-\frac{9}{\left(9-x^{2}\right)^{3 / 2}} f^{\prime \prime}(x)=-\frac{9 x}{\left(9-x^{2}\right)^{3 / 2}}