Chapter 11 - Further Topics in Algebra - 11.2 Arithmetic Sequences and Series - 11.2 Exercises - Page 1025: 55

$3240$

We are looking for the sum of the first 80 positive integers. Use the formula $S_n=\frac{n}{2}(a_1+a_n)$ with $n=80, a_1=1, a_{80}=80$: $S_n=\frac{n}{2}(a_1+a_n)$ $S_{80}=\frac{80}{2}(1+80)$ $S_{80}=40*81$ $S_{80}=\boxed{3240}$