More arithmetic progression

We can directly apply the formula , $S_{n} = \frac{n(n+1)}{2}$ or we can do it the old fashioned way, $S = 1+2+3+.....200 \rightarrow (i)$ $S = 200+199 +198 +......+1 \rightarrow (ii)$ $(i)+(ii),$ $2S = 201 + 201 +201 +..... 201 \text {(200 times)} \Rightarrow 2S = 201 \times 200$ $\Rightarrow S = \frac{200 \times 201}{2} \Rightarrow \boxed {20100}$

$A=1+2+3+\ldots+200$

$2A=201+201+201+\ldots+201 = 201\times200$

$\Rightarrow A=\frac{201\times200}{2}=\boxed{20100}$

(200+1 divided by 2) then 200 multiplied by (100.5) =20100