Lesson: Special Right Triangles

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For the following question, can you please tell me why we are adding root 2 for the perimeter?


The hypotenuse of a right triangle is 10 cm. What is the perimeter, in centimeters, of the triangle?

(1) The area of the triangle is 25 square centimeters.
(2) The 2 legs of the triangle are of equal length.
gmat-admin's picture

Hi santhosh1989,

Here's my step-by-step solution to that question: https://gmatclub.com/forum/the-hypotenuse-of-a-right-triangle-is-10-cm-w...


Hi Brent.

Your solution to this problem makes sense, However, another solution has me puzzled by "Math Revolution"

His solution where he pulls (a +b)^2 just seems weird. I'd like to know how he got to the "root 200" and proceeds to solve for an answer. I know an answer is not needed but the continuation to a solution helps me see learn and solve other problems like this might they come up.


gmat-admin's picture

Yes, Math Revolution's solution (here https://gmatclub.com/forum/the-hypotenuse-of-a-right-triangle-is-10-cm-w...) is somewhat counter-intuitive.

Here's the basic idea:

The perimeter = a + b + c (where c is the right triangle's hypotenuse, and a and b are the lengths of the two legs)

Statement 1 tells us that the area is 25
This means ab/2 = 25, which means ab = 50.

We also know that c = 10

From here, Math Revolution decides to see what (a + b)² evaluates to be.

(a + b)² = a² + 2ab + b²
= (a² + b²) + 2ab

The Pythagorean Theorem tells us that a² + b² = c²
Since c = 10, we can write: a² + b² = 10² = 100

We also know that ab = 50

So, let's take (a² + b²) + 2ab and replace a² + b² with 100 AND replace ab with 50. We get...

(a² + b²) + 2ab = 100 + 2(50)
= 100 + 100
= 200

This means (a + b)² = 200, which means a + b = √200 = 10√2

At this point, we can determine the perimeter.

a + b + c = (a + b) + c
= 10√2 + 10

Does that help?


Thanks Brent !

From here, Math Revolution decides to see what (a + b)² evaluates to be.
(a + b)² = a² + 2ab + b²
= (a² + b²) + 2ab

I understand that we have the "Pythagorean formula and the Area formula" and because of that we now have a system of two equations, so therefore it is solvable, but I'm still confused as to how one would see, or know to take (a + b)² and "square" it? It seems so "Non-Sequitur" and out there for lack of a better term or description. What am I missing, I still don't or wouldn't ever get to do that.
gmat-admin's picture

I wouldn't be concerned about not seeing that particular approach; most people will miss it.

The idea here is that we know that the Pythagorean Theorem tells us that a² + b² = c²

We also know (from statement 1) that ab = 50, which means 2ab = 100

So, we have one piece of information about a² + b² AND we have information about 2ab.

Notice that (a + b)² = a² + 2ab + b²

Since the given information seems similar to the expansion of (a + b)², Math Revolution decided to see where it might take him/her.


Thanks Brent,

Maybe later it will click with me, but for now I will just take your advice and forget about it... however, I most likely won't, Lol!
gmat-admin's picture

Ha - I'm the same way!

In the below problem, how do one deduce the 30-60-90 triangle from the given info. What property am I missing. I could assume it will form a right angle, say 90 and then deduce 30 (180 - 90-60= 30).

Thanks in advance

gmat-admin's picture

Good question!

First, we're told that the ladder forms a 60-degree angle with the ground.

Second, the height of any triangle is the length of the line from the top of the triangle to the base, SUCH THAT this line is perpendicular to the base (see Jeff's diagram here: https://gmatclub.com/forum/a-ladder-of-a-fire-truck-is-elevated-to-an-an...)

Does that help?


Hi Brent,

Could you please help me understand the approach adopted to solve the following question:


gmat-admin's picture

You bet.

Here's my step-by-step solution: https://gmatclub.com/forum/figures-x-and-y-above-show-how-eight-identica...

Please let me know if you have any questions.


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