Q.When the sun's elevation angle decreases from 60° to 30°, the length of the shadow of a vertical rod increases by 40 meters. What is the height of the rod? (a) \(10\sqrt3\) meter (b) \(15\sqrt3\) meter (c) \(5\sqrt3\) meter (d) None of the above
Answer: D
From triangle ABC, with respect to angle ∠ACB: \( \tan 30^\circ = \cfrac{AB}{BC} \) ⇒ \( \cfrac{1}{\sqrt{3}} = \cfrac{AB}{BC} \) ⇒ \( AB = \cfrac{BC}{\sqrt{3}} \) — (i) From triangle ABD, with respect to angle ∠ADB: \( \tan 60^\circ = \cfrac{AB}{BD} \) ⇒ \( \sqrt{3} = \cfrac{AB}{BD} \) ⇒ \( AB = \sqrt{3} \cdot BD \) — (ii) Therefore, \( \cfrac{BC}{\sqrt{3}} = \sqrt{3} \cdot BD \) ⇒ \( BC = 3 \cdot BD \) ⇒ \( BC = 3(BC - 40) \) ⇒ \( BC - 3BC = -120 \) ⇒ \( -2BC = -120 \) ⇒ \( BC = 60 \) Hence, \( AB = \cfrac{BC}{\sqrt{3}} = \cfrac{60}{\sqrt{3}} = \cfrac{60\sqrt{3}}{3} = 20\sqrt{3} \) ∴ The height of the rod is \( 20\sqrt{3} \) meters.
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