OG Diagnostic Quant 48-When do you make logic assumptions?

[VegasMBA]

Hi Guys,
I have been reading Beat the GMAT posts for a few weeks now and love how supportive and responsive everyone is. This is such a great forum.

I am hoping I can get some help from everyone.

I have hit a question that I just can't rationalize for future questions I may have that are asking for similar information.

#48 on the OG Quant Diagnostic is a geometry question asking you to calculate the perimeter and area of the given rectangle. The first support in the DS question offers the diagonal measure. This is enough information to calculate the other two sides of the triangle (or the length and width of the rectangle). The triangle is a right triangle. There are no mysteries where the right angle is in the triangle. I think the information is straightforward enough to use my shortcut knowledge about 3-4-5 right triangles in this case. If the longest measure of a right triangle is 10, then the other two should be 8 and 6 using my knowledge of rectangles properties. Why is this not enough information?

The GMAC eventually builds the same answer in their explanation.

This is one thing I am trying to resolve from question to question. It seems like in some instances you should make logical assumptions to get your answer from both PS and DS questions . Using the information they give you you can take the question further to calculate the final answer...even if it isn't plainly stated. Then in other cases, you have to find your solution using only the information they give you.

What is the rule of thumb when attacking a question regarding stated information?
Or at least how is GMAC expecting me to evaluate the question?

[pepeprepa]

Well, what you say is almost right.
Every triangle with sides ratios which are 3:4:5 --> right triangle. Nevertheless, you cannot say it is a right triangle SO it is 3:4:5.
Just look for right triangles which are 45-45-90 the ratios are x:x:x*sqrt(2) and 30-60-90 the ratios are x:x*sqrt(3):2*x
Indeed, you cannot use this.

Hope I'm right, hope it helps you.
Welcome!

[Ian Stewart]

From Statement 1, all you know is that the hypotenuse of a right triangle has length 10. While the triangle might be a 6-8-10 triangle, there is no way to be sure about this. The triangle could just as well be a 1-sqrt(99)-10 triangle, or a 7-sqrt(51)-10 triangle, among others. Statement 1) would, incidentally, be enough information if you were told the length and width are integers- but that's not something we're told here.

I'm not convinced it's worth bothering to learn the 3-4-5 and 5-12-13 triangles before the GMAT, though I wouldn't be surprised if other instructors disagree. These triangles are only rarely helpful to know about and are very easy to work out quickly anyway, and there's the danger that you might start seeing them when they aren't really there.

[VegasMBA]

Thanks so much for the response.

I still don't think I am grasping why the length of the diagonal using the information given in the question isn't enough to answer the question in number one.
OR
Why should I not make that conclusion?

A rectangle has 4 right angles where two parallel sides are longer than the other two parallel sides they connect with, right?

If that's the case, why wouldn't it be a 3-4-5 triangle?

Or...should I just mark in my notes not to do that on Geometry questions?

I am hoping to find some consistency in the questions, and I guess that is where I am a little dumbfounded... :roll:

[VegasMBA]

Hi Ian,

That explanation makes sense to me.

I am thinking I may need to drop that quick tip off my list...or at least make sure I have the measure off two sides before I get too far.

[pepeprepa]

Here is a similar one from the OG green #109:

l=length
w=width
d=diagonal
That's for a rectangle.
What is the area of the rectangular region?
1) l+w=6
2) d²=20

[pepeprepa]

Another one is #95 of OG green.

[VegasMBA]

Great, I will check those out. Thanks so much pepeprepa!