Proof

This can be proved by mathematical induction.

As 1=12, it is true when n=1.

Assume it is true for n=k, then 1+3+5+...+(2k-1)=k2
add the next item (2k+1).
Then 1+3+5+...+(2k-1)+(2k+1)=k2+(2k+1)=(k+1)2.

Therefore if it is true for n=k it is true for n=k+1.

It is true for n=1 therefore it is true for all values of n.