jmc

Question

For 1, 2, 3, , n and each of its non-empty subsets a unique alternating sum is defined as follows. Arrange the numbers in the subset in decreasing order and then, beginning with the largest, alternately add and subtract successive numbers. For example, the alternating sum for 1, 2, 3, 6,9 is 9-6+3-2+1=5 and for 5 it is simply 5. Find the sum of all such alternating sums for n=7.

Accepted Answer

Let S be a non- empty subset of 1,2,3,4,5,6. Then the alternating sum of S, plus the alternating sum of S 7, is 7. This is because, since 7 is the largest element, when we take an alternating sum, each number in S ends up with the opposite sign of each corresponding element of S 7. Because there are 2^6=64 of these pairs of sets, the sum of all possible subsets of our given set is 64 7, giving an answer of 448.