Question

Find the sum of $90$ terms of A.P $4,8,12,...$

Answer 1

$a=4,d=8-4=4$

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

$S_n=\frac{90}{2}[2\times 4+(90-1)\times 4]$

$=45[4+89\times 2]$

$=45\left[2(2+89)\right]$

$=90\times 91=8190$