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555-605 (Medium)|   Geometry|                        
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The 30-60-90 triangle can be determined by 180(n-2)/n = 120, then splitting this isoceles triangle in 1/2 to find the base.
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AbdurRakib



In the figure shown,PQRSTU is a regular polygon with sides of lenght x.What is the perimeter of triangle PRT in terms of x?


A. (\(x\sqrt{3}\))/2

B. \(x\sqrt{3}\)

C. (3\(x\sqrt{3}\))/2

D. 3\(x\sqrt{3}\)

E. 4\(x\sqrt{3}\)

OG Q 2017(Book Question: 145)

Attachment:
1494909854_591a839e50d63.png


Side of the triangle = \(x\sqrt{3}\)
Perimeter of the triangle =\(3 x \sqrt{3}\)

IMO D

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Senthil1981
Answer is D:

The equilateral triangle inside a regular hexagon of side x will have it's side as \(x*\sqrt{3}\). Therefore perimeter is 3x*\(\sqrt{3}\)


Hi,

is it a property that always holds true ?

Thanks
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AbdurRakib



In the figure shown,PQRSTU is a regular polygon with sides of length x. What is the perimeter of triangle PRT in terms of x?


A. (\(x\sqrt{3}\))/2

B. \(x\sqrt{3}\)

C. (3\(x\sqrt{3}\))/2

D. 3\(x\sqrt{3}\)

E. 4\(x\sqrt{3}\)

OG Q 2017(Book Question: 145)

Attachment:
The attachment 1494909854_591a839e50d63.png is no longer available

The polygon is a regular hexagon, with each angle \(120\): \(\frac{180(6-2)}{6}\)

Thus angle \(Q \) is \(60\) degree

Drawing perpendicular \(QN \) from \(Q\) to \(PR\) we get QNP is a 60-90-30 Triangle \((30-60-90)\) Pythagorean triples

Let, OP is x, so \(QN=\frac{x}{2}\)

\(PN=x\sqrt{\frac{3}{2}}\) and \(NR\) = \(x\sqrt{\frac{3}{2}}\)

\(PR= x\sqrt{\frac{3}{2}}*2=x\sqrt{3}\)

The perimeter \(=x\sqrt{3}*3\)

\(=3x\sqrt{3}\)

The answer is \(D\)
Attachments

Hexahon.png
Hexahon.png [ 4.48 KiB | Viewed 4514 times ]

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HimanshuBahuguna
Consider Triangle PQR
(a) It is an isosceles triangle hence two smaller angles are equal and = (180-120)/2=30 degree.
(b) Third side of the triangle PQR = cosine projections of 'x' = 2*x*cos 30 = (2)X(x)*(Sqrt (3)/2)=(Sqrt 3)x
(c) Same logic holds for other 2 sides, and hence perimeter = 3*(Sqrt 3)x

Why I cannot use sine rule here? Am I missing out on a certain condition to use sine rule? I tried using it. But getting 3x/2 as answer.

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After going through the explanations given here, I thought of posting some tips for those who couldn't find the right answer.
Maybe it would help you to eliminate a few answer options next time and improve your accuracy.

Especially in Geometry, if you are going for a logical guess, the fundamental rules can be used to eliminate some answer choices.

Let's try it out here.
Assume that each side of the regular hexagon be x.
Since all the answer options have x in them, rules connecting the sides of the triangle can be your first choice.

The side opposite to the greatest angle would be the largest in a triangle. I hope, all of you are aware of this fundamental rule.


Each interior angle of a regular hexagon is 120°.
In triangle PUT, side PT will be largest among other 2 sides as 120° would be the greatest angle in this triangle and side opposite to that would be largest. So PT > x, Similarly PR and RT each should greater than x.

Hence, the perimeter of △ PRT = PT + PR + RT should be greater than 3x.

So, the correct answer option should be greater than 3x.

Let us investigate and do some eliminations. :cool:

If you analyze the GMAT answer options, either it would be in ascending or descending order. Here Its in ascending order.

The best choice would be to start with Option C.

C. (3x√3)/2 This is less than 3x. :)

√3/2 is less than 1. When 3x is multiplied by a number less than 1, it will give you a value less than 3x.

Hence, option C is eliminated. Since the answer options are in ascending order, All answer options above C i.e Option A and B will also get eliminated as it's value is less than 3x.

Only option D and option E are left.

Note: It's a regular hexagon i.e All 6 sides, as well as each interior angles, are equal.

Further analyzing the figure we will get to know that △ PRT is an equilateral triangle i.e all sides are equal


In Option E, the perimeter of △ PRT is given as 4x√3. So , each side would be 4x√3/3 = 4x/√3 = 4x/1.7 > 2x

Is it possible to have the side PT > 2x?

In △ PUT, the other two sides, PU and UT are equal i.e x (Properties of the regular hexagon).

Now, it's time for another fundamental rule, Third side in a triangle should be less than the sum of the other 2 sides. Here, PT should be less than x + x i.e 2x. Therefore, Option E is eliminated.

Option D should be the correct answer as it's the only one left.

Always remember, The fundamental properties/rules in geometry can be used to eliminate answer choices when you are stuck.
Above was an attempt to illustrate the same.


Thanks,
Clifin J Francis,
GMAT QUANT SME
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AbdurRakib



In the figure shown,PQRSTU is a regular polygon with sides of length x. What is the perimeter of triangle PRT in terms of x?


A. (\(x\sqrt{3}\))/2

B. \(x\sqrt{3}\)

C. (3\(x\sqrt{3}\))/2

D. 3\(x\sqrt{3}\)

E. 4\(x\sqrt{3}\)

OG Q 2017(Book Question: 145)

Attachment:
1494909854_591a839e50d63.png

Why so much mathhhhhh? Ballpark!!!

UT =x
Is PT longer than UT, the same as UT, or shorter than UT? Longer. Cool, by how much? I don't know, let's go with a little less than twice as long. Fine, so let's call PT something like ~1.8x? Then the perimeter of the triangle is ~3*1.8x, so ~5.4x.

Look at the answer choices to see if there's one that fits for ~5.4x.

Square root of 3 is ~1.7.

A) \(\frac{\sqrt{3}}{2}x\) ... \(\frac{1.7}{2}x\) ... that's way too low ... wrong.
B) \(\sqrt{3}x\) ... 1.7x ...that's way too low ... wrong.
C) \(\frac{3\sqrt{3}}{2}x\) ... \(\frac{(3*1.7)}{2}x\) ... \(\frac{5.1}{2}x\) ... that's way too low ... wrong.
D) \(3\sqrt{3}x\) ... 3*1.7x ... 5.1x ... hmm, sounds good.
E) \(4\sqrt{3}x\) ... 4*1.7x ... 6.8x ... that's too high ... wrong.

Answer choice D.

Or have a great time with your geometry rules. :lol: :lol:

ThatDudeKnowsBallparking
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