200

one way is by realizing that the above sequence is only a sum of 201's a hundred times,which is equal to: 201*100 =20100

$A=1+2+3+\ldots+199+200$

$\Rightarrow 2A= 201+201+201+\ldots+201=\frac{201\times200}{2}$

$\Rightarrow A= 201\times 100=\boxed{20100}$

$1+2+3+..........200=\frac{200 \times 201}{2}=20100$

200 x 201 / 2 = 20100

Sum of n no.s =n(n+1)/2 So... 200(200+1)/2=100(201)=20100

n(n+1)/2.

Standard result for sigma(n).

just use standard sum of arithmatic (1+200).200/2 = 20100