gmatprep - sum question

there's a whole bunch of ways to do this problem.
here are two.

(1) use COLUMNS (probably the easiest way - definitely the smallest numbers):
the first column adds to -1 - 1 - 1 + 7 = 4.
the second column is exactly twice the first column, so it's 2(4).
the third column is exactly 3 times the first column, so it's 3(4).
etc.
so the total of all the columns is 1(4) + 2(4) + 3(4) + ... 7(4)
= (1 + 2 + 3 + ... + 7)(4)
= 28 x 4
= 112

(2) use ROWS
the first row adds to 28.
the second row is -2 times the first row, so it's -2(28).
the third row is 3 times the first row, so it's +3(28).
etc.
so the total of all the rows is 28 - 2(28) + 3(28) - ... + 7(28)
= (1 - 2 + 3 - 4 + 5 - 6 + 7)(28)
= (4)(28)
= 112

done
mm hmm
oh yeah.