What does the plus minus symbol mean?

Answer 1

The plus minus symbol means both plus and minus should be considered; or that
a measurement or estimate may be plus or minus the number after the sign.

For example, a measured temperature may be state as 123 +/- 0.2 C, which means temperature may be between 122.8 C and 123.2 C.

An estimated attendance at an eventmay be given as 2500 +/- 100 , which means attendance should be between 2400 and 2600.

The quadratic formula contains a plus minus sign, meaning that there are two roots to be
obtained and given consideration.

For example, t = 30 +/- 40 which means the answer may mathematically be 70 or - 10 , Realistically, the answer may be either or both numbers.

Answer 2

It means "plus or minus". Here are two examples:

* In the quadratic formula, it means that there are two solutions; you get one if you replace the "plus-minus" sign with a plus, and the other if you replace it with a minus.

* When specifying a measurement, for example 9.1 plus-minus 0.2, it means (in this example) that the error is not expected to be much greater than 0.2.