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The angle of elevation of the top of a building from the bottom and top of a tower are 60 degrees and 30 degrees respectively. If the tower is 20 m high, the height of the building is
A10 m
B20 m
C30 m
D40 m
Answer & Solution
Correct answer: C. 30 m
1. From bottom: tan(60) = b/d, so b = d (square root of 3).
2. From top of tower: tan(30) = (b - 20)/d, so b - 20 = d/(square root of 3).
3. From these: d × (square root of 3) - 20 = d/(square root of 3); solve: d = 10 (square root of 3); b = 30 m.
_Source: NCERT Class 10 Mathematics, Ch 9 "Some Applications of Trigonometry", §9.2_
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