Chapter 12 Sequences and Series - Mixed Review of Problem Solving - Lessons 12.1-12.3 - Page 818: 5

$105$ pieces of Chalk

The general formula for the sum of the nth term of an arithmetic series is given by $S_n= \dfrac{n}{2}(a_1+a_n)$ We know that $a_n=a_1+(n-1)d$ ....(1) Equation (1) gives: $6=15+(n-1)\times (-1)$ This gives: $15-n+1=6$ or, $n=10$ Thus, $S_{10}= \dfrac{n}{2}(15+6)=105$ pieces of Chalk