Question

what is the disadvantage in comparing line segments by mere observation?​

Answer 1

By mere observation, we cannot be absolutely sure about the judgement. When we compare two line segments of almost same lengths, we cannot be sure about the line segment of greater length. ... This is the disadvantage in comparing line segments by mere observation.

Answer 2

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On comparing the lengths of line segments by mere observation the difference may not be obvious for all the pairs of line segments.