Question

What is the sum of 1 2 3 ... 81??

Answer 1

`S=1+2+3+\cdots+79+80+81`

`S=81+80+79+\cdots+3+2+1`


`\implies 2S=(81+1)+(80+2)+\cdots+(2+80)+(1+81)`

`\implies 2S=81(82)`

`\implies S=81(41)=3321`