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A person standing on the bank of a river observes that the angle of elevation of the top of a tree on the opposite bank of the river is $60^{\circ}$ and when he retires 40 meter away from the tree the angle of elevation becomes $30^{\circ}$. The breadth of the river is
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$20 \mathrm{~m}$
$20 \mathrm{~m}$
$\tan 30^{\circ}=\frac{h}{40+b}$
$\Rightarrow \sqrt{3} h=40+b$
$\tan 60^{\circ}=h / b \Rightarrow h=\sqrt{3} b$
$\Rightarrow b=20 m$
$\Rightarrow \sqrt{3} h=40+b$
$\tan 60^{\circ}=h / b \Rightarrow h=\sqrt{3} b$
$\Rightarrow b=20 m$
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