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655-705 (Hard)|   Geometry|                        
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Bunuel
In the figure shown, what is the value of v+x+y+z+w?



(A) 45
(B) 90
(C) 180
(D) 270
(E) 360

Attachment:
The attachment Star.png is no longer available

Definitely a challenging question.

Notice that each angle in the pentagon corresponds to a triangle with two of the stated variables from the stem. So each angle of the polygon can be calculated as 180 - Variable from Angle 1 - Variable from Angle 2.

Now remember that 180(N-2) in which N = # of sides of a polygon is equal to the SUM of the angles in a polygon. So 180(N-2) = 540. Finally we can sum up each of the calculated polygon angles and set THAT equal to 540. So 900-2(v+x+y+z+w) = 540. From here we can calculate that the sum of the variables is equal to 180.
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ToughGraphExample.png [ 19.5 KiB | Viewed 7817 times ]

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I get all the detailed solutions people are posting, but I used a formula I was taught in high school math class.

Sum of the angles x pointed star:

Sum= (x-4) * 180

So in this case:
Sum= (5-4) * 180 = 180

Choice (C)
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The key insight to greatly simplify the calculation in this problem is realising that since we are not given any specific values of any of the angles in this question, and since it is a PS question with one correct answer in all possible cases of the star, the correct answer would hold true irrespective of what the angles of the pentagon in the star, and the corresponding vertices of the star are. Thus, to simplify our calculation, we may assume it to be a regular pentagon.

In such a case, for any polygon with n sides, the sum of the interior angles = (n-2)*180. Therefore, the sum of the angles inside the pentagon = (5-2)*180 = 540. Because we assume it to be a regular pentagon, each individual angle is 108 degree for each interior angle.

Further, since complementary angles on a straight line must sum to 180 degrees, Each of the external adjacent angle (which also represent one of each of the angles of triangle adjacent to the internal pentagon) must be 180-108 = 72.

The sum of the angles inside each of these triangles is 180. Thus each point of the star to be 180-72-72 = 36.

Since the star has 5 points, the sum of the angles of all of the 5 points as required in the question is 5*36 = 180.
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BrentGMATPrepNow can you pls explain in detail how can we make a perfect star assumption and how would its result be true for every other star


BrentGMATPrepNow
Bunuel
The Official Guide for GMAT® Review, 13th Edition - Quantitative Questions Project

In the figure shown, what is the value of v+x+y+z+w?



(A) 45
(B) 90
(C) 180
(D) 270
(E) 360

First, let's consider a PERFECT star.



Notice that the pentagon in the center is a perfect (regular) pentagon, which means ALL 5 angles are equal.
The sum of the angles in an n-sided figure = 180(n-2) degrees
So, the sum of the angles in this 5-sided figure = 180(5-2) = 180(3) = 540 degrees
Since ALL 5 angles are equal, then the measure of each angle = 540/5 = 108 degrees.



Since two angles on a line must add to 180 degrees, we can see that the angles adjacent to the 108-degree angles must equal 72 degrees (since 180 - 108 = 72)



At this point, we can see that we're dealing with 5 triangles, and for each triangle, we know two of the angle measurements.
Since the sum of the angles in a triangle = 180, we know that each missing angle = 36 degrees (180 - 72 - 72 = 36)


So, v + x + y + z + w = 36 + 36 + 36 + 36 + 36
= 180 degrees

Answer: C
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Hi Elite097,

You do NOT actually have to assume that you are dealing with a 'perfect star' to get the correct answer - as long as you follow all of the built-in Geometry rules that are involved (re: the pentagon is 540 total degrees, each line is 180 degrees and each triangle is 180 degrees). The math will just go a bit faster/easier if you choose that all of the similar-looking angles are equal to one another. What we ultimately have in this drawing is a gigantic 'system' of equations - and once you account for all of the 'inner angles' (re: the ones NOT at the 'tips' of the 5 triangles) - what's left (re: V+W+X+Y+Z) will always total 180 degrees.

Here's a bit more detail about why that happens.

The pentagon is 540 total degrees and each of the 10 'lines' that come off of the pentagon is 180 degrees.
This means that total for the 10 lines is (10)(180) = 1800 degrees and 2(540) = 1080 of those degrees are accounted for by the angles in the pentagon (we count each of those pentagon angles twice because each one touches two of the 10 line segments)...

This leaves 1800 - 1080 = 720 degrees in those line segments that are 'inside' the 5 triangles (re: the 'bases' that touch the pentagon).
Since there are 5 triangles, there are 5(180) = 900 total degrees to account for there.
We've accounted for 720 of those degrees already, so the remaining 900 - 720 = 180 degrees are found at the 'tips' of the 5 triangles.

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Changing the style of the shape from irregular to regular doesn’t change the sum of its internal or external angels.

Let’s restructure this figure to be a regular star that holds inside it a regular pentagon, that won’t change the sums of the angels.

A regular pentagon has 108 degrees for each internal angle, and consequently the generated triangles, to complete the star, has Two equal angles of 72 each, hence the third angle is 36.

Sum of the 5 angles is 36*5=180
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Central Angle Theorem is an interesting and straightforward way to solve this. I got an alternative solution - hope its as simple.

Take a triangle for instance. Sum of external angles WHEN ITS SIDES ARE EXTENDED ON ONE SIDE ALONE is 360. So, when each of the triangle's vertices are extended on both sides and those angles angles summed up, it will be 2*360 i.e., 720.

Now, coming back to our question in hand, we have a pentagon at the centre. All 5 vertices are extended both ways on a straight line, so we can say sum of external angles is 720

Now these external angles are parts of the 5 triangles outside the pentagon. We have v,w,x,y,z in these 5 triangles. Summing up the angles in these 5 gives us the equation (v+w+x+y+z)+(pentagon external angles) = 5*180
therefore v+w+x+y+z = 5*180 - 4*180 = 180

Please let me know if there is a flaw in this solution. I hope it works as smooth as easy as the central angle theorem and this enabled me to solve this question in half a minute.
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