Question
Prove that in a triangle, other than an equilateral triangle, angle opposite the longest side is greater than $\frac{2}{3}$ of a right angle.
✓
Answer
Given: In $\triangle\text{ABC, BC}$ is the longest side.
To prove: $\angle\text{BAC}>\frac{2}{3}$ of a right angle, i.e., $\angle\text{BAC}>60^{\circ}$
Construct: Mark a point $D$ on side $AC$ such that $AD = AB = BD$.
Proof: In $\triangle\text{ABD,}$
$\because\text{AD = AB = BD}$ (By construction)
$\therefore\angle1=\angle3=\angle4=60^{\circ}$
Now, $\angle\text{BAC}=\angle1+\angle2=60^{\circ}+\angle2$ but $60^{\circ}=\frac{2}{3}$ of a right angle
So, $\angle\text{BAC}=\frac{2}{3}$ of a right angle + $\angle2$
Hence, $\angle\text{BAC}>\frac{2}{3}$ of a right angle.
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