1. What is the sum of the three interior angles of any triangle?
- A90 deg
- B180 deg
- C270 deg
- D360 deg
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Answer: B) 180 deg
By the Angle Sum Property of a triangle, the three interior angles always add up to 180 deg.
Class 9 Angle Sum of a Triangle — practice questions with answers
Geometry — Lines and Angles · 5 sample questions from the school syllabus. Try each one, then open the answer and the worked solution.
2. One angle of a triangle is 100 deg and the other two are equal. Find each of the equal angles.
- A36
- B40
- C80
- D44
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Answer: B) 40
Sum of equal angles = 180 - 100 = 80 deg. Each = = 40 deg.
3. One angle of a triangle is 90 deg. The sum of the remaining two angles is:
- A45 deg
- B90 deg
- C135 deg
- D180 deg
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Answer: B) 90 deg
Remaining two angles = 180 - 90 = 90 deg.
4. In triangle XYZ, angle X = 90 deg. A line from Z makes the angle Z split so that one acute angle is 3 times the other acute angle of the triangle.
If angle Y is the smaller acute angle, find angle Y.
If angle Y is the smaller acute angle, find angle Y.
- A45
- B20.5
- C24.5
- D22.5
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Answer: D) 22.5
Acute angles sum to 90 deg. Let smaller = y, larger = 3y. y + 3y = 90, 4y = 90, y = 22.5 deg.
5. In triangle ABC, angle B = 90 deg. The bisector of angle A meets BC at D. If angle C = 40 deg, find angle ADB.
- A65
- B59
- C130
- D71
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Answer: A) 65
Angle A = 180 - 90 - 40 = 50 deg. Bisector gives angle BAD = 25 deg. In triangle ABD: angle ADB = 180 - 90 - 25 = 65 deg.
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