Barron's tells me to "estimate" on diagrams - really?

[sherpa07]

So this might be hard to explain here, but I'll give it a wham:

The Barron's New GRE guide seems to say that estimating angle sizes, side lengths on geometric shapes is OK when not given any information. It calls it "TACTIC 2: trust the diagram," whereas it seems more sensible to me (and, I think, the Princeton Review "Cracking" book) to NOT trust the diagram.

Any opinions on this? When given no additional info about angle sizes, etc., should you just estimate some things and make a guess? I think not, but maybe I'm wrong.

[TakeruK]

You can generally estimate it, if for example you see an acute angle (i.e. less than 90 degrees) and you have to decide between say, 40 degrees and 120 degrees. But you really shouldn't estimate it if you have to pick between, say, 40 degrees and 60 degrees.

Strictly speaking, the lengths and angles are not necessarily to scale when these questions are created. However, from looking at them and doing practice tests / real thing, I don't really see any GRE questions where they purposely mislead you by misrepresenting the side lengths. That is, I think they do draw the diagrams pretty much to scale, but they put the disclaimer there so that if there are mistakes, you might not get the right answer by using your superhuman protractor and ruler powers.

Often though, there is a choice between two pretty close angles so that guessing from the diagram doesn't work unless you've done enough work to eliminate some choices already.

But if you have a question where you have to compare the value of, say, an angle x vs. some other value, and the angle x is some angle in a triangle that is drawn a certain way (with no other angle/side information), you should keep in mind that the angle drawn is not the only possible value for x. So in this case, you definitely should NOT try to estimate the angle. Although I don't know if these types of questions are still in the new GRE.