Imagine that you have two lengths, a and b. You can put the two lenghts together to get a + b and you can find the difference in their lengths, |a - b|.
The blue lines are of length a and b (it doesn't matter which is which) and the green line is of length |a-b|. From the diagram a + b -|a - b| is twice the length of the smaller of a and b. Thus
Does this expression work if a or b or both are negative or zero?Cheers,