GMAT question 151

ironsheep

Junior Member
Joined
May 9, 2019
Messages
106
This question is from "GMAT Official Guide 2019 Quantitative Review", it reads "The points R, T, and U lie on a circle that has radius 4. If the length of arc RTU is 4pie/3, what is the length of line segment RU?" (same as in the image). The answer is D, which is 4.

1. I did 8pie minus (4pie/3) and you get (20pie/3), which is none of the answers, but seems logical.

2. I saw a Youtube video:

and looked at the explanation( which is confusing and also in the book) did (4pie/3)/8pie equals x/360. Cross multiply and you get x equals 60 degrees. 8pie is (2 times 4 times pie, circumference of this circle). Then I did 180 - 60 = 120. (120/360) times 8pie equals (8pie/3), which is B.

I also attached an image of their explanation for anyone who wants to try to understand it, but like I said above, it is confusing to me.
 

Attachments

lev888

Full Member
Joined
Jan 16, 2018
Messages
786
Which part of the explanation is confusing? Seems like you understand how they got the angle. Can you clarify the reasons for your subsequent calculations?
 

ironsheep

Junior Member
Joined
May 9, 2019
Messages
106
Which part of the explanation is confusing? Seems like you understand how they got the angle. Can you clarify the reasons for your subsequent calculations?

Why did they get 4. My first calculation makes sense. If you look at the picture of a circle they drew or you can make one, then what you are looking for is the length of the whole circle minus the arc RTU as RU is everything else.

The second method I used and they did too--- says that 8pie is the same as 360 degrees as both equal the whole for a circle( one is whole length and the other is total degrees in a circle), but I don't get why I got the answer wrong since I did 180 - 60 = 120 just like them. I don't really understand why I have to subtract 60 from 180, but they did it, so I did it. They then talked about equilateral triangles(I don't know why they are throwing that in???).

If you can translate their writing for me that would be cool too, but for the GMAT I have 2 minutes per question and I want to prepare for it.

The second method I used is not really from my thinking, but is the result of the Youtube video and looking at the book's explanation --- if anyone wants to clarify it, feel free.
 

lev888

Full Member
Joined
Jan 16, 2018
Messages
786
Read carefully, the question is "what is the length of line segment RU".
 

JeffM

Elite Member
Joined
Sep 14, 2012
Messages
4,112
Why did they get 4. My first calculation makes sense. If you look at the picture of a circle they drew or you can make one, then what you are looking for is the length of the whole circle minus the arc RTU as RU is everything else.


The second method I used and they did too--- says that 8pie is the same as 360 degrees as both equal the whole for a circle( one is whole length and the other is total degrees in a circle), but I don't get why I got the answer wrong since I did 180 - 60 = 120 just like them. I don't really understand why I have to subtract 60 from 180, but they did it, so I did it. They then talked about equilateral triangles(I don't know why they are throwing that in???).

If you can translate their writing for me that would be cool too, but for the GMAT I have 2 minutes per question and I want to prepare for it.

The second method I used is not really from my thinking, but is the result of the Youtube video and looking at the book's explanation --- if anyone wants to clarify it, feel free.
You have completely misunderstood the question.

Yes, it is certainly true that if the length of the arc RTU is \(\displaystyle 4 \pi / 3\), there is another arc UR, which has length \(\displaystyle 8 \pi - ( 4 \pi / 3) = 20 \pi / 3\) because the radius is 4 so the circumference is \(\displaystyle 8 \pi.\)

But you are not asked about that other arc at all. You are asked about the length of the straight line joining R and U.

Now one of the geometric facts that is relevant to this problem and that you are supposed to know and recognize is that, if you are measuring angles in degrees, the ratio of the length of an arc of a circle relative to the circumference of that circle equals the ratio of the angle subtending that arc relative to 360 degrees. Another relevant geometric fact that you are supposed to recognize is that the line subtending any arc except a semi-circle forms the base of an isosceles triangle with the equal sides running from the center of the circle to the end points of the arc. If you don't get this intuitively, sketch the circle with center O, radius 4, line RU subtending arc RTU, and draw the lines OR and OU. So you have triangle ORU. The final relevant geometric fact that you are expected to know is that the base angles of an isoceles triangle are equal. Now you put all this together.

\(\displaystyle a = \text {measure in degrees of } \angle \text { ROU.}\)

\(\displaystyle b = \text {measure in degrees of each base angle.}\)

Two unknowns require two independent equations.

\(\displaystyle \dfrac{a}{360} = \dfrac{\dfrac{4 \pi}{3}}{8 \pi} = \dfrac{4 \pi}{3} * \dfrac{ 1}{8 \pi} = \dfrac{1}{6}.\)

\(\displaystyle a + b + b = 180 \implies b = \dfrac{180 - a}{2}.\)

\(\displaystyle \text {But } \dfrac{a}{360} = \dfrac{1}{6} \implies a = \dfrac{360}{6} = 60 \implies b = 60.\)

So all the angles in the triangle are equal to 60 degrees. So it is what kind of triangle? How does that let you answer the question?

The actual computations in this problem are quite easy. It can be done well within 2 minutes if first you read the problem carefully, second make a crude sketch, and lastly have the relevant geometric facts at your fingertips. This problem tests your knowledge of geometric facts, not computational skill.
 
Last edited:

ironsheep

Junior Member
Joined
May 9, 2019
Messages
106
Now I am more confused, look at the circle they drew in the image. There is no straight line joining R and U. Even if you made one, OR is 4 and OU is 4, but does that mean UR is 4?? This isn't a right triangle, so you can't use A squared + B squared = C squared. Also, UR would be between 0 (4-4) and 8(4+4).
 

JeffM

Elite Member
Joined
Sep 14, 2012
Messages
4,112
Now I am more confused, look at the circle they drew in the image. There is no straight line joining R and U. Even if you made one, OR is 4 and OU is 4, but does that mean UR is 4?? This isn't a right triangle, so you can't use A squared + B squared = C squared. Also, UR would be between 0 (4-4) and 8(4+4).
In the problem as given, they draw no picture at all, but they specifically say "what is the length of line segment RU." If I drew a sketch for a problem about the line RU, I'd include that line in my sketch.

No, this is not a right triangle. So the Pythagorean Theorem is a complete irrelevancy.

But if all three angles in a triangle are equal, what kind of triangle is it?
 

ironsheep

Junior Member
Joined
May 9, 2019
Messages
106
In the problem as given, they draw no picture at all, but they specifically say "what is the length of line segment RU." If I drew a sketch for a problem about the line RU, I'd include that line in my sketch.

No, this is not a right triangle. So the Pythagorean Theorem is a complete irrelevancy.

But if all three angles in a triangle are equal, what kind of triangle is it?

What makes you think all the angles are 60 degrees?
 

JeffM

Elite Member
Joined
Sep 14, 2012
Messages
4,112

ironsheep

Junior Member
Joined
May 9, 2019
Messages
106
Top