The third side of the triangle must be longer than the difference of the other two sides:
x > (8.5 -8.0)
x > 0.5
And it must be shorter than their sum:
x < (8.5 +8.0)
x < 16.5
The third side must be in the range ...
0.5 < x < 16.5
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These limits are a direct consequence of the triangle inequality, which requires the sum of the two shortest sides exceed the length of the longest side.
