Multiple choice

Flag-staff of length $d$ stands on a tower of height $h$. lf at a point on the ground the angles of elevation of the tower and the top of the flag-staff be $\alpha,\ \beta$ respectively, then $h=$

$\displaystyle \dfrac{d\cot\beta}{\cot\beta-\cot\alpha}$
$\displaystyle \dfrac{d\tan\beta}{\tan\alpha-\tan\beta}$
$\displaystyle d[\frac{\tan\alpha+\tan\beta}{\cot\alpha-\cot\beta}]$
$\displaystyle \frac{d\tan\alpha}{\tan\beta}$

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