Answer
500500
Work Step by Step
$\sum ^{1000}_{i=1}i=\dfrac {\left( a_{1}+a_{1000}\right) \times n}{2}\Rightarrow a_{1}=1;a_{1000}=1000;n=1000\Rightarrow S=\dfrac {1+1000}{2}\times 1000=500500$
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