1+ 1=2 is an axiom in mathematics and needs no proof. Because 1+ 1=2 is the basis of all mathematical theorems and cannot be proved mathematically. As for "Why is 1+ 1 equal to 2?" As a problem, we don't need to prove it mathematically. In fact, we only need to explain why 1+ 1=2 can be said to be a definition or an axiom. However, it can be proved by reduction to absurdity that if 1+ 1 is not equal to 2, mathematics is a pot of porridge, and where mathematics is used, human society will be in a mess, then 1+ 1 must be equal to 2.

Another is equal to the field, or equal to three, which means that for the family, one plus one baby for life.