You know that there exists an injective mapping
Basis : for n=2, it's verified (I assume it's a know fact for you)
Inductive hypothesis : assume that there exists an injective mapping
Now, you have to prove that there exists an injective mapping
For this, define this way :
It is easy to show that it's injective, knowing that is injective.
Now prove that there exists an injection from to
Since are countable, there exist injections
Once again, it's easy to show that it's injective.
We know that are injective.
Now you just have to prove that the composite of two injective functions is injective, in particular .
And you're done.