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there is probably an easier way to do this but to see if a point is inside of a circle.

Equation of a Circle: (parametric coordinates)
for a circle with origin (j, k) and radius r:
x(t) = r cos(t) + j y(t) = r sin(t) + k

tes to see if the distance from the center of the circle to the point is longer than the radius of the circle.