Sum of Cross Products

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Given a set of pairs of numbers, the sum of the cross products is computed by taking the product of each pair of numbers and summing these products. For the data,

1 3
4 6
7 9
1 1

the sum of cross products is: 1 x 3 + 4 x 6 + 7 x 9 + 1 x 1 = 91.