Question

What is the sum of first 11 terms of an AP whose middle term is 30? [2 MARKS]

Answer 1

Concept: 1 Mark
Application: 1 Mark

The middle term
$a_6=a+5d=30$

$S_{11}=\frac{n}{2}[2a+(n-1\left)d\right]$

$S_{11}=\frac{11}{2}(2a+10d)$

$=\frac{11}{2}\times 2(a+5d)$

$=11(a+5d)$

$=11\times 30$

$=330$