Checking if a point is inside a circumference?

[lolfoo]

enter radius:
enter x:
enter y:
enter x for the point to test for:
enter y for the point to test for:

So how do i check to see if the point is inside the circunference? Is there any mathematical relation? Like if it was a triangle (areaxyt==areaxp+areayp+areatp)?

[Peter87]

Check if the distance between the point and the center of the circle is less than the radius.