Mathematical Reasoning
A hexagon has 6 sides.
Question 1 · Multiple choice
Question
A hexagon has 6 sides.
How many diagonals can be drawn inside a hexagon?
Options
- A.9
- B.10
- C.12
- D.15
- E.18
Correct answer
Explanation
Step 1: Understand what a diagonal is
A diagonal is a line segment connecting two non-adjacent vertices of a polygon. It goes through the inside of the shape (NOT along the sides).
Step 2: Count diagonals from one vertex
A hexagon has 6 vertices. From any one vertex, you can draw a line to:
- 5 other vertices total
- Subtract 2 adjacent vertices (connected by sides, not diagonals)
- Diagonals from one vertex = 6 − 1 (itself) − 2 (adjacent) = 3 diagonals
Step 3: Calculate the total
6 vertices × 3 diagonals each = 18
But each diagonal is counted twice (once from each end), so divide by 2:
Total diagonals = 18 ÷ 2 = 9
Alternative formula: n(n − 3) ÷ 2 = 6 × 3 ÷ 2 = 9
Answer: 9 diagonals
| Option | Value | Why it is wrong |
|---|---|---|
| A | 9 | 6 × (6 − 3) ÷ 2 = 9. ✓ |
| B | 10 | Off-by-one error. ✗ |
| C | 12 | Forgot to divide by 2 (each diagonal counted twice). ✗ |
| D | 15 | Counted all pairs of vertices including sides: C(6,2)=15 — but sides are not diagonals. ✗ |
| E | 18 | 6 × 3 = 18 — forgot to divide by 2. ✗ |