- A21 m
- B17 m
- C3 m
- D15 m
Step-by-Step Solution
The post and the shadow meet at a right angle, so the ray is the hypotenuse: m.
Answer: D) 15 m
A 9 m lamp post casts a 12 m shadow on level ground. How long is the dashed sun-ray from the top of the post to the tip of the shadow?
Class 7The Triangle and its PropertiesTriangles
Key idea: Pythagoras theorem (right triangles only)
In a **right** triangle, the square of the hypotenuse equals the sum of the squares of the other two sides:
hypotenus = side₁² + side₂²
Classic Pythagorean triple: 3, 4, 5 — because .
Other triples to recognise: (5, 12, 13), (8, 15, 17), (7, 24, 25). The rule only applies when one of the angles is 90°. Never use Pythagoras for non-right triangles.
hypotenus = side₁² + side₂²
Classic Pythagorean triple: 3, 4, 5 — because .
Other triples to recognise: (5, 12, 13), (8, 15, 17), (7, 24, 25). The rule only applies when one of the angles is 90°. Never use Pythagoras for non-right triangles.
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