$a+(a+d)+(a+2 d)+\ldots(a+(n-1) d)=\frac{n}{2}[2 a+(n-1) d]$
Let P(n) be the given statement.
Now,
$P(n): a+(a+d)+(a+2 d)+\ldots+(a+(n-1) d)=\frac{n}{2}[2 a+(n-1) d]$
Step 1:
$P(1)=a=\frac{1}{2}(2 a+(1-1) d)$
Hence, $P(1)$ is true.
Step 2:
Suppose $P(m)$ is true.
Then,
$a+(a+d)+\ldots+(a+(m-1) d)=\frac{m}{2}[2 a+(m-1) d]$
We have to show that $P(m+1)$ is true whenever $P(m)$ is true.
That is,
$a+(a+d)+\ldots+(a+m d)=\frac{(m+1)}{2}[2 a+m d]$
$W e$ know that $P(m)$ is true.
Thus, we have :
$a+(a+d)+\ldots+(a+(m-1) d)=\frac{m}{2}[2 a+(m-1) d]$
$\Rightarrow a+(a+d)+\ldots+(a+(m-1) d)+(a+m d)=\frac{m}{2}[2 a+(m-1) d]+(a+m d)$
[Adding $(a+m d)$ to both sides]
$\Rightarrow P(m+1)=\frac{1}{2}\left[2 a m+m^{2} d-m d+2 a+2 m d\right]$
$\Rightarrow P(m+1)=\frac{1}{2}[2 a(m+1)+m d(m+1)]$
$=\frac{1}{2}(m+1)(2 a+m d)$
Thus, $P(m+1)$ is true.
By the principle of $m$ athematical $i$ nduction, $P(n)$ is true for all $n \in N$.