#### Answer

$$ x^2+y^2=1681$$

#### Work Step by Step

Since the center is at the origin, we use the distance formula to find that the radius must be:
$$\sqrt{\left(9-0\right)^2+\left(40-0\right)^2} \\ =41$$
Thus, it follows:
$$x^2+y^2=r^2 $$
$$ x^2+y^2=1681$$