Just how deceptive can the diagrams be?

[guest]

Hi!

I was under the impression that, while we shouldn't assume much from the visual appearance of a triangle -- in other words, the fact that an angle looks like it's 90 degrees doesn't mean that it actually is 90 degrees -- the test wouldn't do something completely gonzo, like use a picture of a 60-60-60 triangle to represent a 90-3-87 triangle.

The MGMAT geometry question bank question "Unknown Leg" has confused me. The problem includes a visual of a triangle that looks vaguely like a 30-60-90. But one of the explanations asks us to consider the possibility that the 60+/- degree angle is actually much greater than 90 degrees. In this case, the diagram wouldn't be "off," "rough," or "imprecise"; it would be aggressively deceptive. (If you were drawing the triangle with a crayon on a napkin, you wouldn't be able to draw a 120-degree angle in a way that made it appear to be a 60-degree angle.)

Here's the text:

"What is the length of segment BC?

(1) Angle ABC is 90 degrees.

(2) The area of the triangle is 30."

I understand the problem and the explanation; I got the wrong answer only because I assumed that the GMAT wouldn't show use a very acute angle to represent a very obtuse angle, or use an angle that looks like it's 90 degrees to represent, e.g., a 20-degree angle. But the Q bank explanation is making me wonder whether that's true. Can you help?

PS: This forum is a great resource. Thanks!

[Guest]

Hi!

I was under the impression that, while we shouldn't assume much from the visual appearance of a triangle -- in other words, the fact that an angle looks like it's 90 degrees doesn't mean that it actually is 90 degrees -- the test wouldn't do something completely gonzo, like use a picture of a 60-60-60 triangle to represent a 90-3-87 triangle.

The MGMAT geometry question bank question "Unknown Leg" has confused me. The problem includes a visual of a triangle that looks vaguely like a 30-60-90. But one of the explanations asks us to consider the possibility that the 60+/- degree angle is actually much greater than 90 degrees. In this case, the diagram wouldn't be "off," "rough," or "imprecise"; it would be aggressively deceptive. (If you were drawing the triangle with a crayon on a napkin, you wouldn't be able to draw a 120-degree angle in a way that made it appear to be a 60-degree angle.)

Here's the text:

"What is the length of segment BC?

(1) Angle ABC is 90 degrees.

(2) The area of the triangle is 30."

I understand the problem and the explanation; I got the wrong answer only because I assumed that the GMAT wouldn't show use a very acute angle to represent a very obtuse angle, or use an angle that looks like it's 90 degrees to represent, e.g., a 20-degree angle. But the Q bank explanation is making me wonder whether that's true. Can you help?

PS: This forum is a great resource. Thanks!

The best advice I can give you is to only use the stated facts and fact based deductions you can make from them. Don't assume anything is true unless it is stated or can be proven. Remember that this test is gauges your ability to sift through facts and draw provable conclusions. Don't take the bait unless you can prove without a doubt that there's not a hook buried inside of it!

[StaceyKoprince]

On the official test, for problem solving questions, they will attempt to draw pictures to scale. If a particular diagram can't be drawn to scale for whatever reason, the diagram will be labeled with "Note: this picture is not drawn to scale."

On a data sufficiency question, anything goes. If it's not drawn to scale, they won't tell you. If you see a circle with a dot in the middle labeled O, you can assume the figure is indeed a circle and that the dot O is indeed inside that circle somewhere, but that's it. If they really do want you to know that the dot is in the center of the circle, the problem text will tell you so explicitly. If you see a picture of a triangle showing an acute angle, that might be one way of drawing the triangle, but it may not be the ONLY way. If you can also make that an obtuse angle using all of the given information, that's another valid possibility and will possibly change your answer from sufficient to insufficient.

(The problem mentioned above is, in fact, a data sufficiency problem.)