Find the value of \[+1+2+3-4-5-6+7+8+9-10-11-12+\cdots -2020,\]where the sign alternates between $+$ and $-$ after every three numbers.
Problem
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Tags: arithmetic, sums, IMONST
Bowser498
19.09.2020 02:52
For every six numbers, the sum is equal to $-9$. Try grouping the first and fourth terms, second and fifth, and third and sixth and you'll see why. This cycle keeps occurring until we get to the end, which must consist of $...+2018+2019-2020-2021-2022$. We see that there are $337$ of these cycles, so the sum should be $-9(337)=-3033$. However, we counted $-2021$ and $-2022$, which we must add back. The answer is $-3033+2021+2022=\boxed{1010}$.
Mathpluspianoequalslife
19.09.2020 03:14
group every three nums
bobthegod78
19.09.2020 05:08
Every 6, actually
Mathpluspianoequalslife
19.09.2020 05:13
group everfy 2020 nums