Question 992590
x^2 + y^2 = 49


here's the graph of that equation.
{{{graph(600,600,-10,10,-10,10,sqrt(49-x^2),-sqrt(49-x^2))}}}


the vertex form of the eqaution of a circle is:


(x-h)^2 + (y-k)^2 = r^2


r is the radius of the circle..
(h,k) is the (x,y) coordinate of the center of the circle.


in your circle, the center is (0,0) so h = 0 and k = 0
in your circle, 


any point p(x,y) that lies on the circle, will be r units away from the center of the circle.


here's a reference that might  help you understand that better.


<a href = "https://www.mathsisfun.com/algebra/circle-equations.html" target = "_blank">https://www.mathsisfun.com/algebra/circle-equations.html</a>


they use (a,b) as the center of the circle,
i use (h,k) as the center of the circle.
doesn't matter.
they reference the same thing.


a or h references the x-coordinate of the center of the circle.
b or k references the y-coordinate of the center of the circle.