GMAT考试：Testprep数学精解2_GMAT数学

GMAT考试：Testprep数学精解2

　　The above problem is difficult because many people subconsciously assume that there are only two candidates. They then figure that since the challenger　received 49% of the vote the incumbent received 51% of the vote. This would　be a valid deduction if C were the only challenger 。

　　But we cannot assume that. There may be two or more challengers. Hence，　　

　　Now， consider alone. Since Incumbent I received 25，000 of the 100，000 votes cast， I necessarily received 25% of the vote. Hence， the answer to the question is No， the incumbent did not receive over 50% of the vote. Therefore， is sufficient to answer the question. The answer is B.

　　Note， some people have trouble with because they feel that the question　asks for a yes answer. But on Data Sufficiency questions， a no answer is　just as valid as a yes answer. What were looking for is a definite answer.

　　CHECKING EXTREME CASES

　　When drawing a geometric figure or checking a given one， be sure to includedrawings of extreme cases as well as ordinary ones.

　　Example 1： In the figure to the right， AC is a chord and B is a point on the　circle. What is the measure of angle x?

　　Although in the drawing AC looks to be a diameter， that cannot be assumed. All we know is that AC is a chord. Hence， numerous cases are possible， three　of which are illustrated below：