单选题 Which of the following points lies in the interior of the circle whose radius is 10 and whose center is at the origin?

(A) (-9, 4)
(B) (5,-9)
(C) (0,-10)
(D) (10,-1)
(E) (-6, 8)

【正确答案】 A

【答案解析】Find the distance from each point to (0,0), the center of the circle. We're looking for a point that is less than 10 units from the center. The distance from (a,b) to (0,0) equals [*] Check each point. A: (-9,4) [*] [*]＜10 (KEY FACT N2).
[*]
**Clearly, (0,-10) is 10 units from the origin, and so is on the circle. Also, since (10,0) is on the circle, (10,-1) is outside. The others are too close to call without knowing the formula or using the Pythagorean theorem, so if you're not sure, guess.