In the figure shown, the length of line segment QS is

Question

https://gmatclub.com/forum/download/file.php?id=16891 In the figure shown, the length of line segment QS is 4\sqrt{3} . What is the perimeter of equilateral triangle PQR? A. 12 B. 12 \sqrt{3} C. 24 D. 24 \sqrt{3} E. 48 Triangle.png What am i doing wrong here ? PQ^2 = QS^2 + PS^2 PQ^2 = PS^2 + 48 ------ (1) QR^2 = SR^2 + 48 ------(2) Adding 1 and 2 PQ^2 + QR^2 = PS^2 + SR ^2 + 96 ----- 3 But, PS^2 + SR ^2 = PR ^2 ------- 4 Substitute 4 in 3.... PQ^2 + QR^2 = PR^2 + 96 -----5 PQ = PR = QR since equilateral triangle Therefore PQ^2 = 96 PQ = sqrt (96) But this doesnt yield the correct answer ....

BrentGMATPrepNow · Accepted Answer

Let's first add the given information to the diagram. 
If ∆PQR is an equilateral triangle, then the 3 angles are each 60°
 https://i.imgur.com/lQva2Wr.png

At this point, we can see we have a special 30-60-90 right triangle, which we can compare with the  BASE 30-60-90 right triangle  
 https://i.imgur.com/6FdzWzP.png

When we compare the sides that are opposite the 60° angle, we can see that  4√3  is 4 times the value of  √3 
So, the magnification factor is 4. 
This means the other 2 lengths will be 4 times the sides of the BASE 30-60-90 right triangle 
 https://i.imgur.com/995ZrvX.png

Now that we know that each side has length 8, the perimeter = 8 + 8 + 8 = 24

Answer: C

Cheers,
Brent

agdimple333 · Answer

Red bold is not true... PS+SR = PR... so if you square (PS+SR)^2 = PR2... not what you have written

simplest way to solve this problem is sin60 =  \sqrt{3} /2 = QS/PQ = 4 \sqrt{3} /PQ
==> PQ = 8
so perimeter = 24
choice C.

Impenetrable · Answer

agdimple333 is simple and easy. But how do I solve it without knowing sin (60)?

Bunuel · Answer

Trigonometry is not tested on the GMAT, which means that EVERY GMAT geometry question can be solved without it. In the figure shown, the length of line segment QS is 4\sqrt{3} . What is the perimeter of equilateral triangle PQR? A. 12 B. 12 \sqrt{3} C. 24 D. 24 \sqrt{3} E. 48 Triangle.png Since triangle PQR is equilateral then its all angles equal to 60°. So, right triangle QSR is a 30°-60°-90° right triangle (with angle R equal to 60°). Next, in a right triangle where the angles are 30°, 60°, and 90° the sides are always in the ratio 1 : \sqrt{3}: 2 and the leg opposite 60° (QS) corresponds with \sqrt{3} , which makes QR equal to 4\sqrt{3}*\frac{2}{\sqrt{3}}=8 ( \frac{QS}{QR}=\frac{\sqrt{3}}{2} --> QR=QS*\frac{2}{\sqrt{3}}=8 ). Thus the perimeter is 3*8=24. Answer: C. For more on this subject check Triangles chapter of Math Book: math-triangles-87197.html Hope it helps.

saiesta · Answer

Here is how I solved it.

An equilateral triangle has 3 equal sides, thus each angle is 60 degrees. Equilateral PQR has a line right down the middle, dividing the triangle PQR into two separate 30-60-90 triangles.

https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcS9g4Yo9lLrw-iKnj0d9wbzOopA6I1OD8QIS87sRJMIeWXcsoxu

The properties of the 30-60-90 triangles are as follows: 
Side with an angle of 30 degrees has a side length of  x 
Side with an angle of 60 degrees has a side length of  x\sqrt{3} 
Side with an angle of 90 degrees has a side length of  2x

Knowing that  4\sqrt{3}  =  x\sqrt{3}  ----->  4 = x  (the side length of the 60 degree angle), you can easily calculate the rest of the other sides of the triangle.
So, putting it all together the perimeter of Equilateral PQR (see attached figure) =  8 + 8 + 4 + 4 = 24

shasadou · Answer

Height in a an equilateral triangle equals   \sqrt{3} /2 *a , where a is a side of such triangle. Hence a =  \sqrt{3} /2 * a = 4 \sqrt{3}

Abhishek009 · Answer

Altitude of a equilateral triangle is  {\sqrt{3} * side}/2

{\sqrt{3} * side}/2  =  4\sqrt{3} .

So sides are 8

Perimeter of an equilateral triangle is 3* sides = 3*8 =>24

Hence answer is (C)

bumpbot · Answer

Automated notice from GMAT Club BumpBot: 

A member just gave Kudos to this thread, showing it’s still useful. I’ve bumped it to the top so more people can benefit. Feel free to add your own questions or solutions.

 This post was generated automatically.

In the figure shown, the length of line segment QS is

Prep Toolkit

Top 5 GMAT Debrief Videos

615 to 715 in 15 Days (Ria's Strategies)

More than Quant/Verbal Abilities: Speed vs. Accuracy

745 with Only Self-Prep and GMAT Club

From 555 to 765: 210-point Improvement

Perfect 805 Score Debrief by Julia

My Rewards

GMAT Problem Solving (PS) Questions

In the figure shown, the length of line segment QS is

Prep Toolkit

615 to 715 in 15 Days (Ria's Strategies)

More than Quant/Verbal Abilities: Speed vs. Accuracy

745 with Only Self-Prep and GMAT Club

From 555 to 765: 210-point Improvement

Perfect 805 Score Debrief by Julia

My Rewards