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This problem trains for: AMC-10, SAT-II.

Which summation symbol expresses the sum:

equation

Expand the first sum to see that there are some negative terms:

equation

equation

Expand the second sum to see that some of the terms also turn to be negative:

equation

equation

Expand the third sum and notice there are negative terms:

equation

equation

The fourth sum is the same as the proposed one, with the terms re-arranged:

equation