中等MATH-GEOM-005
The circle (x − 1)^2 + (y + 2)^2 = 9 has which centre and radius?
A. Centre (1, 2), radius 9
B. Centre (−1, 2), radius 3
C. Centre (1, −2), radius 9
D. Centre (1, −2), radius 3✓ 答案
解說
The centre is (1, −2), from the opposite of the signs inside the brackets. The radius is √9 = 3, not 9.