The standard form of a circle is [tex](x-h)^2+(y-k)^2=r^2[/tex], where h and k are the coordinates of your center. Your center is (0, 0) and your radius, r, is 15. We will fill in accordingly. [tex](x-0)^2+(y-0)^2=15^2[/tex]. That simplifies down to [tex] x^{2} +y^2=225[/tex]. And you're done!
