Geometry p. 209, task 19, explanation p. 215

[rheyn]

Hey everyone!

As I worked through the Medium Practice set, I stumbled over the explanation for the following question:

In the above diagram, which of the following quantities would be sufficient to solve for the value of angle c? Indicate all such quantities. etc.

One of the correct answers is C (angle f as the opposing angle provides sufficient information to solve for angle c).

However, letter F (angles d+f provide sufficient information) is wrong. Why? If I have sufficient information by one of the angles I can simply omit the other half, can't I?

Looking forward to your answers,
Robert

[tommywallach]

Hey Robert,

If you're unsure about something like this, try plugging in values for d + f. Let's say it adds up to 100. That would tell you that a + c = 100, and so b = 80. However, we could never get a single value for c, which is what the question asked for.

I think what you're mistaking is that it WOULD allow you to solve for angle b. The other mistake you might be making is thinking that a = c or d = f, which we don't actually know.

Let me know if that makes sense.

-t

P.S. Side-note, but you shouldn't assume something is a typo if it doesn't make sense. While our books do have the occasional typo in them, always triple-check the question itself before assuming it's a book error. We work darn hard to clean up those silly things, though they do occasionally creep in!

[rheyn]

Hey Tommy,

Thanks for the explanation. Although I made neither of the errors you mentioned, I finally learned that I misunderstood the task:

You mentioned that you give quantities of angles which made me understand what is written there:

The task would not provide me with specific values for angle d (e.g. 100°) and angle f (40°) but only with the sum of (d+f) (in this case 140°).

Having realised that, it totally makes sense that one cannot compute angle solely with that information.

I hope I made clear what I mean. Being a non-native and having no educational experience, it is sometimes really hard to figure why GRE test makers do not test knowledge but only set traps. Does that make any sense?

Sorry for the typo guess ;)

Robert

PS: Math exercises and fear of physical pain might stimulate the same cerebral structures. Go figure =D

[tommywallach]

Hey Robert,

You make a very good point. The GRE does not test inherent math ability, but instead tests something that is most similar to general intelligence/lateral thinking. In other words, you can't beat the test by simply memorizing concepts; you must be able to apply them in situations that are half-problem and half-riddle. Recognizing this fundamental truth of the GRE is half the battle in conquering it!

-t