Adding the whole numbers from 1 to 100

What is $1+2+3+\cdots+100$? Give also a formula for $1+2+\cdots+n$.

$5050$; in general $\tfrac{n(n+1)}{2}$. Write the sum forwards and backwards and add: each of the $n$ columns totals $n+1$, so twice the sum is $n(n+1)$. For $n=100$ that gives $100\cdot101/2=5050$.