中等MATH-GEOM-010
A point moves so that it is always 3 units from (1, −1). What is its locus?
A. The line x = 1
B. The line y = −1
C. The circle with centre (1, −1) and radius 9
D. The circle with centre (1, −1) and radius 3✓ 答案
解說
A fixed distance from a fixed point is a circle. The radius is the given distance 3, so the equation uses 3^2 = 9 on the right-hand side, not radius 9.