Calculating the perimeter of an acute / obtuse triangle - GMAT Quantitative

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Question

A triangle has 2 sides length 5 and 12. Which of the following could be the perimeter of the triangle?

I. 20

II. 25

III. 30

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Answer

For a triangle, the sum of the two shortest sides must be greater than that of the longest. We are given two sides as 5 and 12. Our third side must be greater than 7, since if it were smaller than that we would have where is the unknown side. It must also be smaller than 17 since were it larger, we would have .

Thus our perimeter will be between and . Only II and III are in this range.

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