Question 13
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How many triangles can be formed from the vertices of a regular decagon if no side of the triangle is a side of the decagon?
- A 50 Correct
- B 120
- C 70
- D 60
Correct Answer: Option A
All triangles: 10C3 = 120
Two sides on the decagon: 10
Exactly one side on it: 10 × 6 = 60
120 − 70 = 50
Trap: 70 is the number removed.
Answer: 50
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