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Congruence of Triangles and Inequalities in a Triangle — MATHS STD 9 — Question

Rajasthan BoardEnglish MediumSTD 9MATHSCongruence of Triangles and Inequalities in a Triangle1 Mark
Question
In $\triangle\text{ABC, BC = AB}$ and $\angle\text{B}=80^{\circ}.$ Then, $\angle\text{A = ?}$
  1. 50°
  2. 40°
  3. 100°
  4. 80°
✓

Answer

  1. 50°
Solution:
In $\triangle\text{ABC,}$
$\text{BC = AB}$
$\Rightarrow\angle\text{A}=\angle\text{C}$ (angles opposite to equal sides are equal)
Now, $\angle\text{A}+\angle\text{B}+\angle\text{C}=180^{\circ}$
$\Rightarrow\angle\text{A}+80^{\circ}+\angle\text{A}=180^{\circ}$
$\Rightarrow2\angle\text{A}+100^{\circ}$
$\Rightarrow\angle\text{A}=50^{\circ}$

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