Question

Question 17
A window of a house is h m above the ground. Form the window, the angles of elevation and depression of the top and the bottom of another house situated on the opposite side of the lane are found to be $\alpha$ and $\beta$, respectively. Prove that the height of the other house is $h(1+tan\alpha cot\beta )m$.

Answer 1

Let the height of the other house = OQ = H

And, OB = MW = x m

Given that, height of the first house = WB = h = MO

And $\angle QWM=\alpha ,\angle OWM=\beta =\angle WOB$

[alternate angle]

Now, in $\Delta WOB,tan\beta =\frac{WB}{OB}=\frac{h}{x}$ (window)

$\Rightarrow x=\frac{h}{tan\beta }\dots \dots \left(i\right)$

And in $\Delta QWM$, $tan\alpha =\frac{QM}{WM}=\frac{OQ-MO}{WM}$

$\Rightarrow tan\alpha =\frac{H-h}{x}$

$\Rightarrow x=\frac{H-h}{tan\alpha }$ ...(ii)


From Eq. (i) and (ii),

$\frac{h}{tan\beta }=\frac{H-h}{tan\alpha }$

$\Rightarrow htan\alpha =(H-h)tan\beta$

$\Rightarrow htan\alpha =Htan\beta -htan\beta$

$\Rightarrow Htan\beta =h(tan\alpha +tan\beta )$

$\therefore H=h\left(\frac{tan\alpha +tan\beta }{tan\beta }\right)$

$=h(1+tan\alpha .\frac{1}{tan\beta })=h(1+tan\alpha .cot\beta )$ $[\because cot\theta =\frac{1}{tan\theta }]$

Hence, the required height of the other house is $h(1+tan\alpha .cot\beta )$