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Explanation:
Consider a triangle with side lengths a,b,c such that a < b < c
a = 3 and b = 19 are the two known sides
Let c be the missing side length
The missing side has the property that...
b-a < c < b+a
19-3 < c < 19+3
16 < c < 22
Meaning that if c is a whole number then it can take on any of these values {17, 18, 19, 20, 21}
So c = 21 is the largest possible third side, where only whole number sides are considered.
We can't have a third side of 22 because the two other sides add to a+b = 3+19 = 22 and a triangle wouldn't form (instead only a straight line would). I recommend cutting out strings of paper trying this out yourself.
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Side note: The inequality b-a < c < b+a is a variation of the triangle inequality theorem.
