Lesson: Tackling Data Sufficiency Questions

Comment on Tackling Data Sufficiency Questions

Why shouldn't we estimate lengths and angles when most figures on the gmat are drawn to scale? Actually, it's a great idea to estimate lengths and angles if you cannot answer the question using traditional methods. Most books on the gmat tell you to do this.
gmat-admin's picture

There are 2 kinds of math questions on the GMAT: Problem Solving questions and Data Sufficiency questions. In a Problem Solving question, the diagram will be drawn to scale unless stated otherwise. In a Data Sufficiency question, the diagrams are not necessarily NOT drawn to scale.

In this video (https://www.gmatprepnow.com/module/gmat-geometry/video/885), we note that (at 0:50) one can use estimation for a Problem Solving question.

In the video above, we deal with strategies pertaining to Data Sufficiency geometry questions. Since the diagrams in Data Sufficiency questions are not necessarily NOT drawn to scale, we advise you to avoid estimation.

Hi Brent,

I get your point on the statements together forcing the line to have only one length. But why should we not check till the point of at least formulating an equation that will find the length?

How can we be sure that the data will give us a length?

Additionally, I'm also curious as to how you use this data to find the length in this specific problem.

Thanks!
gmat-admin's picture

Here's why we don't need an equation: if a statement forces a shape into having just one length, then we COULD just recreate a super accurate representation of the figure (using a compass, ruler, protractor, etc), and then measure the line with a ruler.

In the given diagram, we COULD draw an altitude from point A down to side AC. This altitude will divide the triangle into two equal triangles. Both triangles will be 30-60-90 right triangles with a base of length 5. We can then use what we know about 30-60-90 right triangles to calculate the length of side AB.

Brent,
I am having a hard time understanding the solution to this question.
https://gmatclub.com/forum/if-in-the-figure-above-ad-db-and-de-2-what-is-the-length-of-line-se-224874.html

From your excellent 'lock-in' method, I found the length of AC can take more than one value and this value seems to be dependent on the angle x. However, the question asks if we can find the value of AC. Using statement 2, dont we get more than one value? Shouldnt the answer be C because we are looking for one value of AC? Please clarify.
gmat-admin's picture

Keep in mind that we also know that AD = DB.

So, once we know that DE||AC, we have similar triangles (i.e., ∆ABC ~ ∆DBE), which means ∆ABC is TWICE the size ∆DBE (since AB = 2DB)

Since DE = √2, we can be certain that AC = 2√2

gmat-admin's picture

Keep in mind that we also know that AD = DB.

So, once we know that DE||AC, we have similar triangles (i.e., ∆ABC ~ ∆DBE), which means ∆ABC is TWICE the size ∆DBE (since AB = 2DB)

Since DE = √2, we can be certain that AC = 2√2

Hi Brent,

How is AD= DB? It's AE = AC. Also i didn't got DE || AC.
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