Question

Find the sum of first 3 terms of an APwhose nth term is 2n-1

Answer 1

$\text{Given,}\\T_n=2n-1$
$S_n=\sum{T_n}=\sum{2n-1}\\=\sum{2n}-\sum{1}\\=2\frac{n(n+1)}{2}-n\\=n(n+1)-n\\=n^2$

$\text{now, sum of three terms}\\S_3=(3)^2=9$
$\text{hence, answer is} \: \boxed{\boxed{\bold{9}}}$