比如问X的值
条件一给出X可能是A或者B
条件二给出X可能是A或者C
那请问应该选C还是E？
我看prep里说是C，在费费宝典看到的又是E
PREP具体题目是：
148. 14854-!-item-!-187;#058&010192 In the xy-plane, does the line withequation y = 3x + 2 contain the point (r, s) ? (1) (3r + 2 - s)(4r + 9 - s) = 0 (2) (4r - 6 - s)(3r + 2 - s) = 0 -------------------------------------------------------------------------------------------------------------------------------------------------------- 【答案】C 【思路】 r is the xcoordinate, s is the y, so we want to know whether:
s=3r+2
(1) Essentially, one equation or the other must beequal to zero (if xy = 0, the x = 0 or y = 0). either 3r+2-s=0 or 4r+9-s=0, so: s=3r+2 or s=4r+9
If it is true that3r+2-x = 0, then we see that (r.s) must be included in 3r+2 = x, since we have thatexact equation. However, it is possible that 4r+9 =s, in which case ourequation doesn't have to be 3r+2 = s In the first case, (r.s) falls in the line. In thesecond case, (r,s) does not fall in the line. 无法确定(r,S)会落在哪一条线…..insufficient because it could be the second case (2) either 4r-6-s=0 or 3r+2-s=0, so: s=4r-6 or s=3r+2 In the first case, (r.s) falls in the line. In thesecond case, (r,s) does not fall in the line. 无法确定(r,S)会落在哪一条线…..insufficient because it could be the second case (1)+(2) y=3x+2 will contain (r,s) only if s=3r+2.
(1) s=3r+2 OR 4r+9. Insuff.
(2) s=4r-6 OR 3r+2. Insuff.
Combining: s=3r+2. Suff. C
当(1)、(2)同时存在时，会出现交集，此交集上的任何一点皆可以满足题目s=3r+2，故选C。
费费的题目也差不多，但是解释那里说，因为这三个不同的方程组有三种不同的组合，当组到没有需要的方程时，就不成立了，所以选E
请问到底怎么回事呀！
谢谢大家
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发表于 2016-5-19 16:33:35
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