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Consider a triangular plot ABC with sides AB = 7m, BC = 5m and CA = 6m. A vertical lamp-post at the mid point D of AC subtends an angle 30º at B. The height (in m) of the lamp-post is -

1. 21 2
2. 21 2 3
3. 3 7
4. 21 3 2

Correct answer: 21 3 2

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