Is the ± symbol usable ?

[cerulean ingot]

So basically, when I asked my brother if the ± symbol is usable to say that a solution can be both positive and negative, he told me that although people might understand, it's not right and can't be used in proper mathematics, but I kinda see it everywhere so was just wondering.

[dark totem]

Rigorously speaking, I have been taught not to use it. In reality, in can be used to give a solution, but i would not do any calculations with a ±, just replace it so that it only appears in the result.
The best example is the roots of quadratics :
If i was just asked to give the roots, then i would use the ±. But if I had to work with those, then I would separate the + and the -

[cerulean ingot]

ok thx

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