- A115°
- B65°
- C25°
- D135°
Step-by-Step Solution
An exterior angle equals the sum of the two interior opposite angles: .
Answer: A) 115°
Side BC of triangle ABC is extended to D. If ∠A = 45° and ∠B = 70°, find the exterior angle ∠ACD.
Class 7The Triangle and its PropertiesTriangles
Key idea: Exterior angle property
Extend one side of a triangle past a vertex. The angle formed outside the triangle is the **exterior angle** at that vertex.
**Exterior angle = sum of the two opposite interior angles.**
So if the two non-adjacent interior angles are 50° and 60°, the exterior angle = 50° + 60° = 110°.
Why? The exterior angle and the adjacent interior angle form a linear pair (sum 180°), and the three interior angles also sum to 180°. Subtract: exterior = sum of the other two interior angles.
**Exterior angle = sum of the two opposite interior angles.**
So if the two non-adjacent interior angles are 50° and 60°, the exterior angle = 50° + 60° = 110°.
Why? The exterior angle and the adjacent interior angle form a linear pair (sum 180°), and the three interior angles also sum to 180°. Subtract: exterior = sum of the other two interior angles.
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