CPM Homework Help : A2C Problem 12-83

Find the sum of the first $11$ terms of the sequence $21, 26, 31, 36, 41, …$

Solution:

$t\left(n\right) = 5n + 16$

$S_{11}=\frac{n}{2}(t(1)+t(n))=\frac{11}{2}(21 + 71)$

$S_{11} = 506$