Posted by **coried** on Tuesday, August 11, 2009 at 6:03pm.

find the center (h,k) and radius r of the circle with the given equation:

x^2 + y^2 + 6x - 8y = 56

- Algebra -
**MathMate**, Tuesday, August 11, 2009 at 6:29pm
This can be done by completing the square, namely we group the x² with the 6x term and add a constant to make it a complete square. The same goes for y.

x²+6x+ y²-8y = 56

(x+3)² -9 + (y-4)² -16 = 56

(x+3)² + (y-4)² = 56 + 9 + 16 = 81 = 9²

Thus the equation of the circle is

(x+3)² + (y-4)² = 9²

meaning that the centre is at (-3,4) and the radius is 9.

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