Mathematical Reasoning
The perimeter of an isosceles trapezium is 30 cm, the length of one lateral side is 5 cm and the height is 4 …
Question 1 · Multiple choice
Question
The perimeter of an isosceles trapezium is 30 cm, the length of one lateral side is 5 cm and the height is 4 cm. Find its area.
Options
- A.40 cm²
- B.50 cm²
- C.60 cm²
- D.80 cm²
- E.36 cm²
Correct answer
Explanation
Step 1: Understand the shape
An isosceles trapezium has two parallel sides (top and bottom) and two equal slanted sides (the lateral sides).
←─── a ───→
┌───────────┐
5 / ↕ 4 \ 5
/ \
└─────────────────┘
←── b ──→
Perimeter = a + b + 5 + 5 = 30 cm
Step 2: Find the sum of the two parallel sides
Using the perimeter formula:
Perimeter = top + bottom + 2 × lateral side
30 = (top + bottom) + 2 × 5
30 = (top + bottom) + 10
top + bottom = 30 − 10 = 20 cm
Step 3: Apply the trapezium area formula
Area = ½ × (top + bottom) × height
= ½ × 20 × 4
= 40 cm²
Answer: 40 cm²
| Option | Value | Why it is wrong |
|---|---|---|
| A | 40 cm² | ½×20×4=40. ✓ |
| B | 50 cm² | Subtracted only one lateral side: (30−5)×4÷2=50. ✗ |
| C | 60 cm² | Used full perimeter as sum of bases: ½×30×4=60. ✗ |
| D | 80 cm² | Forgot the ½: 20×4=80. ✗ |
| E | 36 cm² | Misread height as 3.6 instead of 4: ½×20×3.6=36. ✗ |