Infinity is not a number. So, you cannot add or subtract it. It's more likely that the quantity you refer to as infinity is an expression that gets larger than any number you can give.. We use the terms infinity and - infinity not as a number but to say that it gets arbitrarily large. Negative infinity means that it gets arbitrarily smaller than any number you can give. so 1 - infinity = -infinity and 1 + infinity = + infinity makes sense only when looked as in this sense. Addition and subtraction are operations that are only defined for real numbers (or some other algebraic structure) and infinity is not a real number.
