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I have some problem to make the Underbrace partly for the equation like the picture shows. The unique part is the "b" is used twice.

enter image description here

Thanks !

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What is the expected output? Try to explain it a little better. EDIT: I mean, if we achieve that “b is used twice”, both underbraces will obviously collide. Do you want one under, and other over? And then, on the top, the bigger one? – Manuel Feb 27 '14 at 10:32
As your reader, I'd be confused by this... – Jubobs Feb 27 '14 at 11:19
@Jubobs ,my bad,Eglish is not my first language. I didn't explain it in a proper way. I would like to say that the first under-brace is for "a+b" and the second under-brace is for "b+c" but there is only one "b" appeared in the equation. And usually I use \underbrace{a+b}_\text{bla bla} for the first under-brace. However, the b is already used buy the first under-brace so \underbrace{b?+c}_\text{bla bla} is really a problem for me – SLN Feb 27 '14 at 11:40
No no :) I understand what output you want to achieve. I'm just saying I have a problem with the output itself: whether b belongs to one, both or none of the the underbraces is not immediately obvious to me, and it may not be to your readers. – Jubobs Feb 27 '14 at 11:46
up vote 10 down vote accepted

You can underbrace “a+(nothing)”, print a zero width “b” and again underbrace “(nothing)+c”:


enter image description here

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Thanks bro! That's what I need!!! – SLN Feb 27 '14 at 11:41
@user3004698 It would require some more work in case the symbol in the middle is wider. Just add to your question in case you need some adjustments. – egreg Feb 27 '14 at 11:43

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