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题库 / OG2017Q-DS-234
In the xy-plane, point (r,s) lies on a circle with center at the origin. What is the value of r2 + s2 ?
(1) The circle has radius 2.
(2) The point(~$\sqrt{2}$~,-~$\sqrt{2}$~) lies on the circle.
Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient.
Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient.
BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient.
EACH statement ALONE is sufficient.
Statements (1) and (2) TOGETHER are NOT sufficient.
题目分析：
点(r,s)位于以原点为圆心的圆上，有~$r^2+s^2=R^2$~，其中R为圆的半径。
（1） 条件1给出了圆的半径，所以~$r^2+s^2=R^2=4$~，题目得解。
（2） 点~$(\sqrt{2},-\sqrt{2})$~也满足~$r^2+s^2=R^2$~，~$(\sqrt{2})^2+(-\sqrt{2})^2=R^2$~，半径R=2，所以~$r^2+s^2=R^2$~，题目得解。
因此，本题答案为（D），任何一个条件说明均能解题。
point (r,s) lies on a circle指点在圆的线上，with center at the origin圆心为原点(0,0)