The midpoint of a line is the halfway mark of that line. A radius measures the distance from a circle's middle point, or origin, to its surrounding perimeter, also known as its circumference. The midpoint has much in common with the radius, for the midpoint on a diameter measures its corresponding radius since the diameter's length is twice that of its radius. You can find the radius of a circle from the coordinates of its diameter's midpoint and the coordinates of a point on its circumference.
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Subtract the xcoordinate of the point on the circumference from the xcoordinate of the midpoint, and then square the difference. For example, the point on the circumference is (3,4) and the midpoint is (7,7). Subtracting the circumference point's xcoordinate with a value of 3 from the midpoint's xcoordinate with a value of 7 results in 4. The square of 4 is 16.

Subtract the ycoordinate of the point on the circumference from the ycoordinate of the midpoint and then square the difference. For this example, subtracting the circumference point's ycoordinate with a value of 4 from the midpoint's ycoordinate with a value of 7 results in 3, and 3 squared is 9.

Add the squares from steps 1 and 2 together and then calculate the square root of that sum to calculate the length of the radius. For this example, 9 added to 16 equals 25, and the square root of 25 is 5. The length of the radius is 5.