GMAT考试-Testprep数学精解(13)
标题： （GMAT）Testprep充分性精解
发信站： BBS水木清华站（Fri Oct 12 16：07：05 2001）
Data Sufficiency
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INTRODUCTION DATA SUFFICIENCY
Most people have much more difficulty with the Data Sufficiency problems tha
n with the Standard Math problems. However， the mathematical knowledge and s
kill required to solve Data Sufficiency problems is no greater than that req
uired to solve standard math problems. What makes Data Sufficiency problems
appear harder at first is the complicated directions. But once you become fa
miliar with the directions， you'll find these problems no harder than standa
rd math problems. In fact， people usually become proficient more quickly on
Data Sufficiency problems.
THE DIRECTIONS
The directions for Data Sufficiency questions are rather complicated. Before
reading any further， take some time to learn the directions cold. Some of t
he wording in the directions below has been changed from the GMAT to make it
clearer. You should never have to look at the instructions during the test.
Directions： Each of the following Data Sufficiency problems contains a quest
ion followed by two statements， numbered （1） and （2）。 You need not solve the
problem； rather you must decide whether the information given is sufficient
to solve the problem.
The correct answer to a question is
A if statement （1） ALONE is sufficient to answer the question but statement
（2） alone is not sufficient；
B if statement （2） ALONE is sufficient to answer the question but statement
（1） alone is not sufficient；
C if the two statements TAKEN TOGETHER are sufficient to answer the question
， but NEITHER statement ALONE is sufficient；
D if EACH statement ALONE is sufficient to answer the question；
E if the two statements TAKEN TOGETHER are still NOT sufficient to answer th
e question.
Numbers： Only real numbers are used. That is， there are no complex numbers.
Drawings： The drawings are drawn to scale according to the information given
in the question， but may conflict with the information given in statements
（1） and （2）。
You can assume that a line that appears straight is straight and that angle
measures cannot be zero.
You can assume that the relative positions of points， angles， and objects ar
e as shown.
All drawings lie in a plane unless stated otherwise.
Example：
In triangle ABC to the right， what is the value of y？
（1） AB = AC
（2） x = 30
Explanation： By statement （1）， triangle ABC is isosceles. Hence， its base an
gles are equal： y = z. Since the angle sum of a triangle is 180 degrees， we
get x + y + z = 180. Replacing z with y in this equation and then simplifyin
g yields x + 2y = 180. Since statement （1） does not give a value for x， we c
annot determine the value of y from statement （1） alone. By statement （2）， x
= 30. Hence， x + y + z = 180 becomes 30 + y + z = 180， or y + z = 150. Sinc
e statement （2） does not give a value for z， we cannot determine the value o
f y from statement （2） alone. However， using both statements in combination，
we can find both x and z and therefore y. Hence， the answer is C.
Notice in the above example that the triangle appears to be a right triangle
…… However， that cannot be assumed： angle A may be 89 degrees or 91 degrees，
we can't tell from the drawing. You must be very careful not to assume any m
ore than what is explicitly given in a Data Sufficiency problem.
ELIMINATION
Data Sufficiency questions provide fertile ground for elimination. In fact，
it is rare that you won't be able to eliminate some answer-choices. Remember
， if you can eliminate at least one answer choice， the odds of gaining point
s by guessing are in your favor.
The following table summarizes how elimination functions with Data Sufficien
cy problems