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The story of what happens if we defined, for example, 1/0 = 0 is a little more complicated. I will include it because it is sufficiently interesting.

Equality of fractions is defined differently when zero divisors are allowed in the denominator. A zero divisor is a non-zero number that can multiply with another non-zero number to get zero. For example, if we work mod 12 then 3 is a zero divisor because 3·4 = 0.

If we want to allow zero or zero divisors in our denominators then we say that a/b = c/d if and only if there is some value s such that s·(a·d - b·c) = 0 where s is anything allowable in the denominator. If we are working with the integers, including this s term does nothing because s has to be something that can be a denominator and we only allow non-zero denominators.

So, even if we define 1/0 = 0 then literally every fraction would be equal to every other fraction.

These conventions can be broken (like, for example, addition of floating point numbers is not associative as pointed out by other comments) but it is definitely not "natural". In other fields of mathematics, like measure theory, it is possible to define things like "zero times infinity is zero" which is traditionally undefined but is a convenient shortcut and does not break anything that people working in measure theory care about.

For more: https://en.wikipedia.org/wiki/Localization_of_a_ring#For_gen...






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