Center and Radius from Circle Equation

Move the constant and complete each square: (x² + 8x + 16) + (y² − 2y + 1) = −1 + 16 + 1, so (x + 4)² + (y − 1)² = 16. The center is (−4, 1) and the radius is √16 = 4. Remember to add the same 16 and 1 to both sides. TI-36X Pro tip: store the center (for x: type (−)4, press sto→, tap the x yzt abcd key until x shows, press enter; store 1 to y the same way), then evaluate x² + y² + 8x − 2y + 1. The result is −16, so r² = 16 and r = 4. This check works for any circle written as x² + y² + Dx + Ey + F = 0.