DSQ: Calculating the length of the side of a rectangle - GMAT Quantitative

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Question

Given parallelogram with diagonal . Is this parallelogram a rectangle?

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Answer

The length of one diagonal alone does not prove the parallelogram to be a rectangle, nor do the lengths of the sides.

Suppose we know all of these lengths, though. Since is a parallelogram, if , then .

The sides and diagonal form a triangle with sidelengths 25, 60, and 65. The parallelogram is a rectangle if and only if is a right angle; therefore, we must determine whether the conditions of the Pythagorean Theorem hold:

This is true; is a right angle and is a rectangle.

Therefore, both statements together are sufficient to answer the question, but neither statement alone is sufficient to answer the question.

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