Equation for given circle. a(4,7) is at center top of circle and b(7,4) is at center right hand side

Note:
The equation for a circle with center at (h,k) and radius r is
(x-h)² + (y-k)² = r²

For the specified circle,
h=4, h=7.

From the diagram shown below, the radius, r, is determined from the Pythagorean Theorem as
r² = 3² + 3² = 18

Therefore the equation for the circle is
(x - 4)² + (y - 7)² = 18

 (x - 4)² + (y - 7)² = 18

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