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Homework Help: Short easy question, what does this sort of plus minus symbol mean?

  1. Oct 3, 2011 #1
    I'm studying for a quantum exam and there is this symbol that looks like an inverted plus minus but the minus is attached to the plus...
    kind of like

    + but attached.
    in context:

    "x ± vt = constant or x = (-/+) + constant."

    Thanks for any speedy help!
  2. jcsd
  3. Oct 3, 2011 #2
    Found an answer on wikipedia:

    There is another character, the minus-plus sign (∓) which is seen less often. It is generally used in conjunction with the "±" sign, in such expressions as "x ± y ∓ z", which can be interpreted as meaning "x + y − z" or/and "x − y + z", but not "x + y + z" nor "x − y − z". The upper "−" in "∓" is considered to be associated to the "+" of "±" (and similarly for the two lower symbols) even though there is no visual indication of the dependency.
    Thanks anyways.
  4. Oct 3, 2011 #3
    isn't this introductory mathematics? :) when solving a quadratic equation you get this plus-minus sign all the time.
  5. Oct 3, 2011 #4
    A good example of the use of the minus plus symbol is the following trig identity:

  6. Oct 3, 2011 #5
    ah that came out pretty small... if you click it, it gets much larger!
  7. Oct 4, 2011 #6


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    Staff: Mentor

    Larger, yes. But strangely, illegible. It shows as the negative of the photo, here. Oh, well then, I guess I'll just never know. :cry: :cry:
  8. Oct 4, 2011 #7
    How odd! I'll write it here then:

    [itex]cos(\alpha ± \beta) = cos \alpha cos \beta \mp sin \alpha sin \beta [/itex]

    All it's saying is that when you use the plus on the LHS, you use the minus on the right and vice-versa.
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