Answer:
1584
Explanation:
The sum of this sequence can be found a number of ways. One way is to recast it as the series whose terms are groups of three terms of the given series.
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series of partial sums
The partial sums, taken 3 terms at a time, are
1+2-3 = 0
4+5-6 = 3
7+8-9 = 6
...
97+98-99 = 96
So the original series is equivalent to ...
0 +3 +6 +... +96 = 3×1 +3×2 +... +3×32 = 3×(1 +2 +... +32)
That is, the sum is 3 times the sum of the consecutive integers 1..32.
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consecutive integers
The sum of integers 1..n is given by the equation ...
s(n) = n(n+1)/2
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series sum
Using this to find the sum of our series, we find it to be ...
series sum = 3 × (32)(33)/2 = 1584
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Alternate solution
The given series is the sum of integers 1-99, with 6 times the sum of integers 1-33 subtracted. That is, ...
1 + 2 - 3 + 4 + 5 - 6 = 1+2+3+4+5+6 -2(3 +6) = 1+2+3+4+5+6 -6(1+2)
Continuing on to ...97 +98 -99 gives the result s(99) -6s(33).
Computed that way, we find the sum to be ...
(99)(100)/2 -6(33)(34)/2 = 4950 -3366 = 1584