A level Maths.
Induction.
Write down the centre and radius of the given circle x^2+y^2=4
I have not come across quadratic equations for radii
This question is in the wrong forum, incidentally. You should post this question in either geometry or algebra.
The general equation for a circle of radius$\displaystyle r$ centered at the point $\displaystyle (a,b)$ is $\displaystyle (x-a)^{2}+(y-b)^{2}=r^{2}.$ From this, can you read off the coordinates of the center, and the radius?