How can I reverse then sign of one number?
In the strict sense, to reverse the sign of any number, then multiply by -1. That will reverse the positive sign to a negative sign, and a negative sign to a positive sign.
If, on the other hand, you only want to reverse negative numbers, that is you always want a positive number, then both foada and swi have provided the best solution for that. Good Luck
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As a circle of light increases so does the circumference of darkness around it. - Albert Einstein