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I would like to know whether you think writing the following expression is considered acceptable:

$$ 3x +-9 $$

Do writing $+-9$ as a way to express $+(-9)$ considered acceptable?

I'm thinking about it as writing $+9$, but replacing $9$ with the scalar of $-9$.

It feels odd doing it, because it's using two operators in sequence, but when you think of -9 as a scalar, it feels fine.

Thank you.

Steph
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    Don't bother about this. Just write $3x - 9$. – Eduardo Longa Jun 25 '16 at 16:28
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    It certainly can be parsed (although $3x + (-9)$ would be more common), but why wouldn't you just write $3x - 9$? – anomaly Jun 25 '16 at 16:28
  • Because sometimes when I solve problems, I write + and then realize the expression is negative, and then I have to erase and replace with -. I have always done that since I was taught that way at school, but now I'm thinking why shouldn't I write +-9 as -9 is a scalar? – Steph Jun 25 '16 at 16:29
  • If $3x+-9$ is acceptable ? No ! You have choice between $3x-9$ or $3x+(-9)$, but certainly not $3x+-9$ – Surb Jun 25 '16 at 16:31
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    I use a trick: I first write just $-$ and if I realize that the coefficient comes with a $+$ sign I just add a $\mid$. – AdLibitum Jun 25 '16 at 16:32
  • In my childhood arithmetic, $\div$ was a subtraction sign -- inline division was $:$ -- so when I need to change a $+$ to a $-$, I just change the crossbar into two fat dots. – hmakholm left over Monica Jun 25 '16 at 16:35
  • As a general rule of thumb, if you feel compelled to ask if your notation is clear, it isn't clear. – Mike Pierce Jun 25 '16 at 16:35
  • @MikePierce The alternative here is writing $3x - 9$, which could also be less clear. In this case the historical happenstance of having $-$ work double duty is to blame for the lack of clarity, not any decision by the question-asker. – schuelermine Aug 29 '22 at 16:51

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If you write $3x+-9$, people will probably know what you are talking about, but this is usually more acceptable: $$3x+(-9)$$ These parentheses make it more clear that the $-$ is a negative sign on the scalar $-9$ while the $+$ is the operation signifying the addition of the terms $3x$ and $-9$.

Noble Mushtak
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  • Both look odd. The first could easily be understood as $3x\pm9$ and the second is puzzling because it is emphasising the minus sign and the reader wonders why. – almagest Jun 25 '16 at 16:33
  • @almagest I do this all the time to emphasize the fact that I'm substituting $-9$ into an expression like $3x+a$. – Noble Mushtak Jun 25 '16 at 16:35
  • Sure. That is because you want to make a point. So if the context is right, fine. But in general it looks to me like something to be avoided. – almagest Jun 25 '16 at 16:37