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.