Question

What is the mathematical induction of 1+3+5+...+(2n-1)=n^2

Answer 1

Let $S(n)$ be the statement that

$1+3+5+\cdots+(2n-1)=\displaystyle\sum_{k=1}^n(2k-1)=n^2$

Check to see that $S(n)$ holds for $n=1$:

$\displaystyle\sum_{k=1}^1(2k-1)=1=1^2\implies S(1)\text{ is true}$

Now suppose that $S(N)$ is true, and use that assumption to show that $S(N+1)$ must also be true. When $n=N+1$, you have

$\displaystyle\sum_{k=1}^{N+1}(2k-1)=\sum_{k=1}^N(2k-1)+2(N+1)-1$
$=N^2+2(N+1)-1$
$=N^2+2N+1$
$=(N+1)^2$

and so $S(N+1)$ is also true, thus proving the statement is true for all $n$.