UPSC CDS Math Basic operation and Factorisation Questions

Considering minus sign, there are total 8 numbers. If we take just two numbers, for example, 1 and 4, then the possible combinations of products are 1×4,−1×4,1×−4,−1×−4
Sum of all these combinations of products =4–4–4+4=0
Similarly, the sum will be zero for all the combinations of products for the remaining numbers.
Thus, the required sum = 0.