Evaluate the Integral by Changing to Polar Coordinates $$\iint _ { D } e ^ { - x ^ { 2 } - y ^ { 2 } } d A$$

Short Answer

Evaluate the integral by changing to polar coordinates.
$$\iint _ { D } e ^ { - x ^ { 2 } - y ^ { 2 } } d A$$
$D$ is the region bounded by the semicircle $x = \sqrt { 9 - y ^ { 2 } }$ and the $y$ -axis.