A more complex question has rarely been asked. Principia Mathematica took nearly a thousand pages to prove that 1+1=2. It does meander a bit, but had they wanted to prove 1+1=2 alone, it could have done so in 500 pages.

Mathematically speaking, the definition of 1 is:

There exists a number such that when multiplied upon an element of a specified set, yields the element of the specified set.
It is also defined as:

1.0000000000000000000000…
.9999999999999999999999999…
as the set of all singletons.
a singleton is a set with exactly 1 element.
These 4 definitions work in tandem with one another.

For example:

1=1

Divide both sides by 3.

1/3=1/3

Rewrite.

1/3=.33333333333333333...

Multiply both sides by 3.

1=.9999999999999999999...

Similarly:

If x =.9999999999999999999...

10x=9.99999999999999999...

10x=9+.99999999999999...

10x=9+x

Simplify by subtracting x from both sides.

9x=9

x=1

.99999999999999999999...=1

As the set of all singletons, 1 is also THE element that represents the set of all single entities.

That is to say: if you have 7 erasers. What you really have is a set of 7 single entities. The definition of 7 becomes: 1 + 1 + 1 + 1 + 1 + 1 + 1; and not as is commonly believed as: 6 + 1.

There is an argument for 7 to be defined as 6 + 1, but this argument is a corollary of the Peano Axioms which in turn argues that there exists a set with absolutely nothing in it {} and a set with exactly something in it {x}. More on this later.
The Principia Mathematica uses Peano's (from the Peano Axioms mentioned earlier) work and notation to expertly slice through the many nuances pertaining to this question.

This is something we will not do; but hopefully, we will also be able to effectively demonstrate why 1 + 1 = 2 in less than 1000 pages.

We will assume these basic principles of number theory:

There exists a number such that when multiplied to an element of a specific set, yields that element of the specific set.
There exists a number such that when added to an element of a specific set, yields that element of the specific set.
If we again assume to have only two sets, a set that is empty: {} containing no elements, and a set that is not empty {x} containing an element. We realize that Consequently, we went from nothing {}, to something {x}. This means that {x} is the successor to {}, as the next step up from nothing, is something.
(Complex way of doing it lol.)