The sum of the first 121 natural numbers is given by the formula:
Sum=n(n+1)2\text{Sum}=\frac{n(n+1)}{2}Sum=2n(n+1)
For n=121n=121n=121:
Sum=121×1222=121×61=7381\text{Sum}=\frac{121\times 122}{2}=121\times 61=7381Sum=2121×122=121×61=7381
The unit digit of this sum is therefore 1
