中等MATH-GEOM-006
What is the centre of the circle x^2 + y^2 − 6x + 4y − 12 = 0?
A. (−3, 2)
B. (3, 2)
C. (3, −2)✓ 答案
D. (−6, 4)
解說
In x^2 + y^2 + Dx + Ey + F = 0 the centre is (−D/2, −E/2). Here D = −6 and E = 4, so the centre is (3, −2). Completing the square gives the same point.