what is the unit digit of sum of first 121 natural numbers?

The unit digit is 1.

Quick Scoop

To find the unit digit of the sum of the first 121 natural numbers, use the formula for the sum of the first nnn natural numbers:

S=n(n+1)2S=\frac{n(n+1)}{2}S=2n(n+1)​

Here, n=121n=121n=121, so:

S=121×1222S=\frac{121\times 122}{2}S=2121×122​

Now focus only on the unit digits:

• Unit digit of 121 is 1
• Unit digit of 122 is 2
• Multiply: 1×2=21\times 2=21×2=2 → unit digit of 121×122121\times 122121×122 is 2
• Then divide by 2: 22=1\frac{2}{2}=122​=1

So, the unit digit of the sum is 1.

Therefore, the unit digit of the sum of the first 121 natural numbers is 1.