Answer:
1.5 < x < 34.7
Explanation:
You want a compound inequality describing possible lengths (x) for the third side of a triangle with sides 18.1 and 16.6.
Triangle inequality
The shortest side cannot be any shorter than the difference of the two known sides:
18.1 -16.6 = 1.5
The longest side cannot be any longer than the sum of the two known sides:
18.1 +16.6 = 34.7
The possible lengths of the third side are given by ...
1.5 < x < 34.7
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Additional comment
If the third side is exactly equal to the sum or the difference, then the "triangle" looks like a line segment. It has zero area. That possibility is usually disallowed. If you want to allow it, then change < to ≤ in the inequality.