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A geometry test asks a student to find the length of two pairs of line segments and then instructs the student to

tell whether they are congruent for each pair.

Is the student obligated to tell if a pair is not congruent, or only if the pair is congruent?

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2 Answers

I think you would have your escape clause if the teacher asked you to tell if they are congruent. For example, "Can you tell me if that timer goes off?".

But whether implies (at least) two alternatives, both of which you're obligated to tell.

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I think that's because whether implies whether or not, which people often say. You don't really hear if or not (although for both words, the or not can be tacked on after the proposition being queried). –  FumbleFingers Sep 21 '11 at 3:14
    
ODO Usage note: If and whether are more or less interchangeable in sentences like I’ll see if he left an address; and I’ll see whether he left an address, although whether is generally regarded as more formal and suitable for written use. –  z7sg Ѫ Sep 21 '11 at 9:59
    
We had a recent question on the "if...or if/whether...or whether" constructions, @z7sgѪ, and I agree they are more or less equivalent. But is there not an additional sense of "if" when used without a list, where it means "if and when"? While substituting "whether" would be translated "whether or not"...That is what I was referring to in my example about the timer going off. –  JeffSahol Sep 21 '11 at 14:35
    
Yes there is an additional sense of if in your example, meaning "in the event that" and whether never has this sense. Substituting whether would be less ambiguous. However, I think in this case, the obtuse schoolboy (or -girl) should just answer the damn question! –  z7sg Ѫ Sep 21 '11 at 16:02
    
I think it's @EdR himself trying to get out of answering the question, which is why I am cutting him no slack here. :) –  JeffSahol Sep 21 '11 at 16:07
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Often whether can expanded to imply whether this... or not thereby demanding an answer irrespective of it being in affirmative or negative.

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