Question 527390
It's easy if you can picture this situation:
The 2 sides that you are given are connected 
with a hinge that has complete freedom.
With the ends in constant contact with a table, start spreading them
until both sides are flat on the table.
You can see that the maximum length of the 3rd side
is the sum of the 2 given sides.
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Now bring the ends toward eachother while keeping them
touching the table. What you will find is the 3rd side
is the difference of the 2 given sides.
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What if the given sides are equal?
Then as you draw them together, you end up with zero for
the 3rd side, which is exactly the difference of the given sides.