Shortcut Trick
The angle bisectors of ∠B and ∠C meet at the incenter O inside ΔABC.
Using the standard incenter angle formula:
∠BOC = 90° + ∠A ÷ 2
⇒ 122° = 90° + ∠A ÷ 2 ⇒ ∠A ÷ 2 = 32° ⇒ ∠A = 64°
∴ The correct answer is 64°.
Alternate Method
Given:
In ΔABC, the bisectors of ∠B and ∠C meet at point O.
∠BOC = 122°
Formula Used:
Sum of angles in a triangle is 180°.
Angle bisector divides an angle into two equal halves.
Calculations:
Let the angles of ΔABC be ∠A, ∠B, and ∠C.
Since OB and OC are the angle bisectors of ∠B and ∠C respectively:
⇒ ∠OBC = ∠B ÷ 2
⇒ ∠OCB = ∠C ÷ 2
In ΔOBC, the sum of angles is 180°:
⇒ ∠OBC + ∠OCB + ∠BOC = 180°
⇒ ∠B ÷ 2 + ∠C ÷ 2 + 122° = 180°
⇒ (∠B + ∠C) ÷ 2 = 180° − 122°
⇒ (∠B + ∠C) ÷ 2 = 58°
⇒ ∠B + ∠C = 58° × 2 = 116°
In ΔABC, the sum of angles is 180°:
⇒ ∠A + ∠B + ∠C = 180°
⇒ ∠A + 116° = 180°
⇒ ∠A = 180° − 116°
⇒ ∠A = 64°
Option Evaluation:
⇒ Option (1) 72°: If ∠A = 72°, then ∠BOC = 90° + 72° ÷ 2 = 126°, which does not match the given 122°.
⇒ Option (2) 64°: If ∠A = 64°, then ∠BOC = 90° + 64° ÷ 2 = 122°, which perfectly matches the given 122°.
⇒ Option (3) 62°: If ∠A = 62°, then ∠BOC = 90° + 62° ÷ 2 = 121°, which does not match the given 122°.
⇒ Option (4) 68°: If ∠A = 68°, then ∠BOC = 90° + 68° ÷ 2 = 124°, which does not match the given 122°.
∴ The correct answer is 64°.
Additional Information
Incenter of a Triangle
The point of intersection of the internal angle bisectors of a triangle is called the incenter. It is the center of the circle that touches all three sides of the triangle internally.
Excenter Angle Formula
If the external bisectors of ∠B and ∠C meet at an excenter E outside the triangle, then the angle formed is ∠BEC = 90° − ∠A ÷ 2.
Angle Bisector Theorem
An angle bisector of an angle of a triangle divides the opposite side into two segments that are proportional to the other two sides of the triangle.