For any number, n, |n| is positive.
So, if n is positive, then n/|n| is +1 and if n is negative, then n/|n| is -1.
For xyz/|xyz|:
-- if all three, x, y, and z, are negative, then xyz is negative, and xyz/|xyz| = -1.
-- if two of them are negative, then xyz is positive, and xyz/|xyz| = +1.
-- if one of them is negative, then xyz is negative, and xyz/|xyz| = -1.
-- if none of them is negative, then xyz is positive, and xyz/|xyz| = +1.
If x is negative, y is negative, and z is negative,
then x/|x| + y/|y| + z/|z| + xyz/|xyz| = -1 + -1 + -1 + -1 = -4
If x is positive, y is negative, and z is negative,
then x/|x| + y/|y| + z/|z| + xyz/|xyz| = +1 + -1 + -1 + +1 = 0
If x is negative, y is positive, and z is negative,
then x/|x| + y/|y| + z/|z| + xyz/|xyz| = -1 + +1 + -1 + +1 = 0
If x is negative, y is negative, and z is positive,
then x/|x| + y/|y| + z/|z| + xyz/|xyz| = -1 + -1 + +1 + +1 = -2
If x is positive, y is positive, and z is negative,
then x/|x| + y/|y| + z/|z| + xyz/|xyz| = +1 + +1 + -1 + -1 = 0
If x is positive, y is negative, and z is positive,
then x/|x| + y/|y| + z/|z| + xyz/|xyz| = +1 + -1 + +1 + -1 = 0
If x is negative, y is positive, and z is positive,
then x/|x| + y/|y| + z/|z| + xyz/|xyz| = -1 + +1 + +1 + -1 = 0
If x is positive, y is positive, and z is positive,
then x/|x| + y/|y| + z/|z| + xyz/|xyz| = +1 + +1 + +1 + +1 = 4