Let an obtuse triangle ABC be given with an obtuse angle B. Angle ABC> 90.
The sum of the inner angles of a triangle is 180, then the sum of the angles (BAC + BCA) = (180 – (> 90)) = <90.
Since the sum of the two angles is less than 90, then these angles are acute, therefore, there cannot be less than two acute angles in a triangle, which was required to be proved.
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