to get the number of answer. it similar to get:
±10 ±20 ±30 ±40 ±50 ±60 ±70 = ±80
to simplify. we are doing this first:
±1±2±3±4±5±6±7=8
There will be 1 group with positive, the other group with negative.
sum of (1 to 7) is 28. and the difference of two groups is 8.
so x+y=28 and x-y =8.
X=18 and Y=10.
To simplify, we only need to consider the sum equal to 10, the combinations are:
(1,2,3,4), (1,2,7),(1,3,6),(1,4,5),(2,3,5),(3,7),(4,6)
so 7 possiblities for the answer of 8, same thing, 7 possibility for the answer of -8. So there are 14 possible solutions.
