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Perimeter questions on the GMAT are a subset of a subset — a small part of the geometry you will inevitably run into on the Quantitative side of the test.  Perimeter is the distance around a geometric figure — from Ancient Greek περίμετρος, or “measure around”.  And on the GMAT it is just that:  the sum of the side lengths of a two-dimensional figure (three dimensional figures have surface area instead).  They are in the category of “easy things made hard”.  It is not challenging to add up the side lengths of a polygon, even an irregular one, so the GMAT will force you to infer more information  to answer the question.

This is a short series of articles showing some of the ways different figures’ information can be used to create perimeter questions.

Last time, I gave you this question about triangles:

In the figure above, triangle CDE is an equilateral triangle with side length 3, AF = 10, EF = 6, and AB = DE.  What is the perimeter of the figure?

(A) 45

(B) 36

(C) 29

(D) 24

(E) 16

The answer is (C) 29.  How?

The question tells us this much:

From here, we can deduce that:

1.  Triangle AEF is a 3:4:5 triangle, with side lengths 6:8:10, respectively.

2.  Since AE is 8 and CE is 3, that makes AC = 5.

3.  Since AC = 5, triangle ABC is also a 3:4:5 triangle, and BC = 4

We can then have:

10+6+4+(3*3) = 29.  Answer choice (A) 45 is the sum of the perimeters of each of the triangles, rather than the perimeter of the figure.

Next time:  putting it all together in complex, mixed figures!

Read other articles in this series:

• Finding Your Way Around GMAT Perimeter Questions (Part 1 of 8 )
• Finding Your Way Around GMAT Perimeter Questions (Part 2 of 8 )
• Finding Your Way Around GMAT Perimeter Questions (Part 3 of 8 )
• Finding Your Way Around GMAT Perimeter Questions (Part 4 of 8 )
• Finding Your Way Around GMAT Perimeter Questions (Part 5 of 8 )
• Finding Your Way Around GMAT Perimeter Questions (Part 6 of 8 )