Determine whether Point A (-5,8) lies on the circle whose center is Point C (1,2) and which contains the Point P (7,-4).
(x-1) 2 + (y-2) 2 = r 2
r 2 = (distance PC) 2
r 2 = (-4-2) 2 + (7-1) 2 = 36 + 36 = 72
so
(x-1) 2 + (y-2) 2 = 72
Now just plug in x=-5 and y=8 and see if it makes the equation true.
If it does then A is on the circumference of the circle.
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