TITLE: How to know if a point is inside a circle?
QUESTION [64 upvotes]: Having a circle with the centre $(x_c, y_c)$ with the radius $r$ how to know whether a point $(x_p, y_p)$ is inside the circle?

REPLY [3 votes]: If you have the equation of the circle, simply plug in the x and y from your point (x,y). After working out the problem, check to see whether your added values are greater than, less than, or equal to the r^2 value. If it is greater, then the point lies outside of the circle. If it is less than, the point is inside the circle. If it is equal, the point is on the circle.