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Octagon

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Source: — Data Sufficiency |
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by Stuart@KaplanGMAT » Sat May 15, 2010 7:59 pm
Rashturi wrote:In the Regular Octagon above BF is parallel to CE, what is the length of the side of the Octagon.

1. Length of BF = 1000

2. Length of CE = 70
Step 1 of the Kaplan Method for DS: analyze the stem

We know our shape is a regular octagon; "regular" shapes have equal angles and equal sides. For example, a "regular quadrilateral" is square; a "regular triangle" is equilateral.

Since we know that our shape is regular, if we know any measurement of the octagon, we can answer any question about the shape.

Step 2 of the Kaplan Method for DS: evaluate the statements

(1) gives us a measurement: sufficient.

(2) gives us a measurement: sufficient.

Each statement is sufficient alone: choose (D).
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by Stuart@KaplanGMAT » Sat May 15, 2010 8:03 pm
As an aside, I think that's an impossible shape (i.e. I don't think those measurements can form a regular octagon) - what's the source of the question?
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by student22 » Sat May 15, 2010 8:11 pm
Stuart, as an aside, let's say that this was a problem solving question (and a regular octagon). How would you go about find the length of a side?
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by Stuart@KaplanGMAT » Sat May 15, 2010 8:45 pm
student22 wrote:Stuart, as an aside, let's say that this was a problem solving question (and a regular octagon). How would you go about find the length of a side?
You wouldn't - that's WAY beyond the scope of the GMAT.

The nice thing about DS is that you don't need to know how to do the calculation, you just need to know that it's possible to do so.
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by Rashturi » Mon May 17, 2010 9:31 am
Stuart Kovinsky wrote:
student22 wrote:Stuart, as an aside, let's say that this was a problem solving question (and a regular octagon). How would you go about find the length of a side?
You wouldn't - that's WAY beyond the scope of the GMAT.

The nice thing about DS is that you don't need to know how to do the calculation, you just need to know that it's possible to do so.
Thanks Stuart, I got this question from one of the forums. But how did u know that they are both sufficient, if I were to see a similar question for pentagon hexagon or an optagon, what rule do I apply to find out whether this is sufficient or not.
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by Stuart@KaplanGMAT » Mon May 17, 2010 11:32 am
Rashturi wrote: Thanks Stuart, I got this question from one of the forums. But how did u know that they are both sufficient, if I were to see a similar question for pentagon hexagon or an optagon, what rule do I apply to find out whether this is sufficient or not.
As a DS question it's definitely inside the scope of the GMAT - it would just be way too time consuming as a PS question.

As long as you were told that the shape were a "regular" pentagon, or hexagon, or whatever, any concrete piece of information would be sufficient to answer almost any question about the shape.

The one thing you have to remember is that if the question asks about a measurement of the shape, you need at least one measurement to even consider sufficiency. Just knowing angles and side proportions is not sufficient (since from angles and proportions we could have a tiny little shape or a huge one).

Here are some examples:

What's the area of the regular hexagon in the diagram below?

(Assume that we have a picture of a hexagon with no information filled in.)

1) Side DE of the hexagon has length 5... sufficient.
2) Angle CDE measures 120 degrees... insufficient.
3) Diagonal AD has length 8... sufficient.
4) The length of side DE plus the length of side EA is twice the length of side BC... insufficient.

It's extremely unlikely that you will see a 5 or greater sided shape on the GMAT that isn't "regular" (except in multiple figure questions, in which case you can divide a weird looking shape into two or more simple shapes).
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