- AObtuse
- BAcute
- CReflex
- DStraight
Step-by-Step Solution
Acute < 90°, right = 90°, obtuse is between 90° and 180°, straight = 180°, reflex between 180° and 360°.
Answer: A) Obtuse
An angle whose measure lies strictly between 90° and 180° is classified as which type?
Class 9Lines and AnglesGeometry — Lines and Angles
Key idea: Exterior angle theorem
**Exterior angle**: extend one side of a triangle past a vertex; the angle outside is the exterior angle.
**Theorem**: The exterior angle of a triangle equals the sum of the two **opposite interior** angles.
Proof: Let interior angles be ∠A, ∠B, ∠C. Extend BC to D forming exterior ∠ACD.
- ∠ACD + ∠ACB = 180° (linear pair)
- ∠A + ∠B + ∠C = 180° (angle sum)
- Subtract: ∠ACD = ∠A + ∠B. ✓
Useful corollary: an exterior angle is always GREATER than either non-adjacent interior angle (since they're positive).
**Theorem**: The exterior angle of a triangle equals the sum of the two **opposite interior** angles.
Proof: Let interior angles be ∠A, ∠B, ∠C. Extend BC to D forming exterior ∠ACD.
- ∠ACD + ∠ACB = 180° (linear pair)
- ∠A + ∠B + ∠C = 180° (angle sum)
- Subtract: ∠ACD = ∠A + ∠B. ✓
Useful corollary: an exterior angle is always GREATER than either non-adjacent interior angle (since they're positive).
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