Using induction to show associativity on $x_1+dots + x_n$

I want to use induction to show that the sum $x_1 + \dots + x_n$ of real numbers is defined independently of parentheses to specify order of addition.

I know how to apply induction(base, assumption, k+1 applying inductive hypothesis). Here I am not sure what the base would be. I have two ideas:

1) First case is $(x_1 + x_2)+x_3+\dots+x_n$ and work through to $x_1+x_2+\dots+x_{n-2} + (x_{n-1} + x_n)$

2) Start with $(x_1+x_2)+x_3=x_1+(x_2+x_3)$ and work up in number of elements to the full case.

Both seem wrong, I have no idea what to actually do.

I imagine above is sufficient effort, although I have shown no working. Before you downvote, please tell me why you are planning it, and I will edit.