This is an example of a "quantitative comparison" question the GRE would test.
Suppose the following information is known:
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one side of a triangle has length 12
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the perimeter of the triangle is 40
What is greater, the area of the triangle or 72?
Either (A) the area of the triangle is greater, (B) 72 is greater, (C) the two quantities are equal, or (D) the relationship cannot be determined from the information given.
Here is my strategy, which may be inefficient given time constraints of the GRE.
First, I eliminate (A) because I know I can make the area of the triangle as close to $0$ as desired, simply by making one of the angles of the triangle as close $0$ as desired. For the same reason, I can eliminate option (C).
Now I wish to show that the area of the triangle could possibly be greater than $72$. If I cannot, then choice (B) is correct. This is where I ask your help — how can one easily find the maximum area of a triangle when only the length of one side and the perimeter is known?
Thank you!
Best Answer
In your case, the area of the triangle can be made to be greater than $72$ if the two remaining sides both have length $14.$ That is, if the triangle is $12-14-14,$ then it is isosceles, so we can find its area pretty easily: Drop an altitude to the $12$-length side, dividing that side into two segments of length $6.$ Then the altitude has length $\sqrt{14^2-6^2} = \sqrt{196-36} = \sqrt{160} = 4\sqrt{10},$ by the Pythagorean theorem, so the area of this triangle is $\dfrac12 (12)(4\sqrt{10}) = 24\sqrt{10},$ which is about $75.89.$
In general, I believe that the area of a triangle given one side and the perimeter is maximized when the two remaining sides have the same length. This should be provable algebraically using Heron's formula.