A circle with center (h, k) and radius r is (x − h)² + (y − k)² = r². Center (3, −2) and radius 5: (x − 3)² + (y + 2)² = 25. Circle questions as a set are in circle problems.
Read h, k, and r
The signs flip: (x − 3) means the center is at x = 3. (y + 2) means y = −2. The number on the right is r squared, so take a square root for the radius. If the equation is expanded, complete the square on x and on y until it looks like this form.
Completing the square is in completing the square. Area and circumference from r are in circumference and area. Formulas as a set are in ACT Math formulas.
The bottom line
Center is (h, k). Radius is the square root of the right side.
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