Question 1104639
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Do you need some sort of formal proof?<br>
The statement is rather obvious.<br>
"2/3 of a right angle" means 60 degrees.
An equilateral triangle has all angles 60 degrees.
If a triangle is not equilateral, then at least one of its angles is greater than 60 degrees.
If BC is the longest side, then the triangle is not equilateral.
The largest angle is opposite the longest side.
Angle A is opposite side BC.<br>
Therefore, if BC is the longest side, then angle A is greater than 60 degrees.