Algebra 2 (1st Edition)

$-403$
The sum of the first $n$ terms of an arithmetic sequence can be obtained by the following formula: $\frac{n(a_1+a_n)}{2},$ where $a_1$ is the first term, $a_n$ is the nth term and $n$ is the number of terms. The nth term of an arithmetic sequence can be obtained by the following formula: $a_n=a_1+(n-1)d$, where $a_1$ is the first term and $d$ is the common difference. Hence here: $d=-3,n=26,a_1=25-3(1)=22,a_{26}=25-3(26)=-53$ Thus the sum:$\frac{26(22-53)}{2}=-403$