Sum of first n natural numbers formula

In this section, we will establish a few useful formulas related to the sum of the natural numbers. These formulas are often used in various competitive exams.

Sum of first n natural numbers:

The sum of consecutive natural numbers is given by the following formula:

Prove that:
$1+2+3+\cdots+n$ $=\frac{n(n+1)}{2}$

Proof: Note that $1, 2, 3, …, n$ form an arithmetic progression (AP) with the first term $a=1$ and with the common difference $d=1.$

By the sum formula of an AP, we have

$1+2+3+…+n$ $=\frac{n}{2}[2a+(n-1)d]$

$=\frac{n}{2}[2.1+(n-1).1]$

$=\frac{n}{2}[2+n-1]$

$=\frac{n(n+1)}{2}$

SOLVED PROBLEMS

Problem 1: Find the sum of the first 100 natural numbers.

Solution: We have to find the sum 1+2+…+100

In the above formula of the sum of first n natural numbers, we put n=100

$\therefore$ the sum $=\frac{100(100+1)}{2}$

$=50 \times 101$

$=5050$

Problem 2: Find the sum of the first 50 natural numbers.

Solution: We have to find the sum 1+2+…+50

In the formula $1+2+3+\cdots+n$ $=\frac{n(n+1)}{2},$ we put n=50

$\therefore$ the sum $=\frac{50(50+1)}{2}$

$=25 \times 51$

$=1275$

Problem 3: In a similar way, the sum of the first 20 natural numbers is

$1+2+3+\cdots +20$

$=\frac{20(20+1)}{2}$

$=10 \times 21=210$

Sum of first n even natural numbers:

Prove that:
$2+4+6+\cdots+2n$ $=n(n+1)$

Proof: Note that $2+4+6+…+2n$ $=2(1+2+3+…+n)$

$=2 \times \frac{n(n+1)}{2}$

$=n(n+1)$

SOLVED PROBLEMS

Problem I: Find the sum of the first 100 even natural numbers.

Solution: We have to find the sum 2+4+…+98+100

In the above formula of the sum of consecutive even natural numbers, we put 2n=100. So n=50.

$\therefore$ the sum $=50(50+1)$

$=50 \times 51$

$=2550$

Sum of first n odd natural numbers:

Prove that:
$1+3+5+\cdots+(2n+1)$ $=n^2$

Proof: Note that $1, 3, 5, \cdots,$$(2n-1)$ forms an AP with $n$ terms. The first term is $a=1$ and the common difference is $d=2.$

$\therefore$ the sum $=1+3+5+$ $\cdots+(2n-1)$ $=\frac{n}{2}[2a+(n-1)d]$

$=\frac{n}{2}[2.1+(n-1).2]$

$=\frac{n}{2} \times 2n$

$=n^2$

SOLVED PROBLEMS

Problem A: Find the sum of the first 100 odd natural numbers.

Solution: We have to find the sum 1+3+…+97+99

In the above formula of the sum of consecutive odd natural numbers, we put 2n-1=99. So 2n=100, that is, n=50.

$\therefore$ the sum of the first 100 odd natural numbers is $50^2$ $=2500.$

FAQs on Sum of Natural Numbers Formula

Q1: What is the sum of the first n natural numbers?

Answer: The formula for the sum of the first n natural numbers is n(n+1)/2.

Q2: What is the formula for the sum of first n even numbers?

Answer: The sum of the first n even numbers is n(n+1).

Q3: Find the sum of the first n odd numbers.

Answer: The sum of the first n odd numbers is $n^2.$