Answer
$648$
Work Step by Step
$a_{n+1}-a_n=3.6(n+1)-18-(3.6n-18)=3.6$
Since the difference between consecutive terms is constant, the given sequence is arithmetic.
The formula for the finite sum of the arithmetic sequence is:
$S_{n}=\frac{n(a_{1}+a_{n})}{2}$
$a_{n}=3.6n-18$
$a_{1}=3.6(1)-18=-14.4$
.
$a_{24}=3.6(24)-18=68.4$
$S_{24}=\frac{24(a_{1}+a_{24})}{2}=\frac{24(-14.4+68.4)}{2}=648$
