Is area of triangle ABC greater than area of triangle DEF?

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by ceilidh.erickson » Tue Oct 13, 2015 2:55 pm
The wording on this question is a little bit strange - we shouldn't be able to directly compare a 2-dimensional area to a 1-dimensional perimeter. But - here's the logic behind this question....

Target question: is the area of ABC > area of DEF?

(1) The value of area of ABC is less than that of perimeter of DEF.


Here, test different values to try to prove insufficiency.

Scenario 1: ABC is an isosceles right triangle with a base of 1 and a height of 1, thus an area of 1/2. DEF is an isosceles right triangle of base 10 and height 10, thus an area of 50 and a perimeter of 20 + 10(sqrt2). DEF has both the greater perimeter and the greater area, so the answer is NO.

Scenario 2: ABC is an isosceles right triangle with a base of 2 and a height of 2, thus an area of 2. DEF is an isosceles right triangle of base 1 and height 1, thus an area of 1/2 and a perimeter of 2 + (sqrt2). DEF has the greater perimeter, but ABC has the greater area, so the answer is YES.

Insufficient.

(2) Angles of ABC = Angles of DEF

If all angles are the same, the triangles are similar, but that doesn't tell us anything about scale. Insufficient.

(1) & (2) Together:

In the examples we used in statement 1, all of the triangles were 45-45-90, but that did not allow us to compare area to perimeter. Insufficient.

The answer is E.
Ceilidh Erickson
EdM in Mind, Brain, and Education
Harvard Graduate School of Education