Question

Find the sum of $45$ terms of A.P $23,21,19,...$

Answer 1

$a=23,d=21-23=-2$

$S_n=\frac{n}{2}[2a+(n-1)d]$

$S_{45}=\frac{45}{2}[2\times 23+(45-1)\times -2]$

$S_{45}=45[23-44]$

$S_{45}=45\times 21=945$

$\therefore {45}^{th}$ term of the A.P is $945$