Respuesta :

I think you just need examples, and not to prove that it is true..

For positive integers, we might do :
3 + (4 + 5) = (3+4) + 5
3 + 9 = 7 + 12

For negative integers we could do:
-3 + (-2 + -1) = (-3 + -2) + -1
-3 + -3 = -5 + -1
-6 = -6