GMAT Math : DSQ: Calculating the height of an equilateral triangle

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Example Question #331 : Geometry

Given equilateral triangles  and , construct the altitude from  to  on , and the altitude from  to  on .

Which, if either, of   and  is longer? 

Statement 1: 

Statement 2: 

Possible Answers:

BOTH statements TOGETHER are insufficient to answer the question. 

Statement 2 ALONE is sufficient to answer the question, but Statement 1 ALONE is NOT sufficient to answer the question.

Statement 1 ALONE is sufficient to answer the question, but Statement 2 ALONE is NOT sufficient to answer the question.

BOTH statements TOGETHER are sufficient to answer the question, but NEITHER statement ALONE is sufficient to answer the question.

EITHER statement ALONE is sufficient to answer the question.

Correct answer:

Statement 1 ALONE is sufficient to answer the question, but Statement 2 ALONE is NOT sufficient to answer the question.

Explanation:

Let  and  be the common side lengths of   and . The length of an altitude of a triangle is solely a function of its side length, so it follows that the triangle with the greater side length is the one whose altitude is the longer. Therefore, the question is equivalent to which, if either, of  or  is the greater.

Assume Statement 1 alone. This statement can be rewritten as

It follows that  has the greater side length, and, consequently, that its altitude  is longer than .

Assume Statement 2 alone.  divides the triangle into two congruent triangles, so  is the midpoint of ; therefore, . Statement 2 can be rewritten as

This statement is inconclusive. Suppose —that is, each side of  is of length 1. Then , and  all make that inequality true; without further information, it is therefore unclear whether , the side length of , is less than, equal to, or greater than , the side length of . Consequently, it is not clear which triangle has the longer altitude.

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