Explanation
In ΔDPC
tanp=CDCP
tan300
1√3=CDCP
CP=CP√3 (1)
IN ΔAPB
tanp=ABPB
tan600=ABBP
√3BP=AB
CD=√3BP (2)
FROM (1) & (2)
CP√3=√3BP
CP=3BP
NOW ,BC=BP+CP
80=BP+3BP
BP=20m
CP=60m
CD=√3BP
=20√3
Let the distance between point of observation and foot of tower BC=20\,m\,
Height of tower AB=H
Angle of elevation of top of tower \theta ={{60}^{0}}
Now from figure \Delta ABC
We know that, \Delta ABC is a right angle triangle.
\tan \theta =\dfrac{AB}{BC}
\tan {{60}^{0}}=\dfrac{h}{20}
\Rightarrow \dfrac{h}{20}=\sqrt{3}
\Rightarrow h=20\sqrt{3}\,\,m.
{\textbf{Step - 1: Find tan }}\alpha
{\text{H = height of the tower}}
\alpha {\text{ = from bottom of a pole the angle of elevation the top of a tower}}
\beta {\text{ = subtends angle}}
{\text{Let AO = BM = a and AB = OM = h}}{\text{.}}
\therefore {\text{PM = H - h}}
{\text{In }} \Delta {\text{AOP}}
{\text{tan}}\alpha {\text{ = }}\dfrac{{{\text{PO}}}}{{{\text{AO}}}}
\Rightarrow {\text{AO = Hcot}}\alpha
\Rightarrow {\text{BM = Hcot}}\alpha \;\;......{\text{(i)}}
{\textbf{Step -2: Find the height of the tower}}{\text{.}}
{\text{In }}\Delta {\text{PBM}},{\text{tan}}(90 - \alpha + \beta ) = \dfrac{{{\text{BM}}}}{{{\text{PM}}}}
\Rightarrow {\text{BM = PM tan}}(90 - \alpha + \beta )\;{\text{ }}\;{\text{ }}\;{\text{ }}\;{\text{ }}\;{\text{ }}\;{\text{ }}\;\;
= {\text{(H - h)cot}}(\alpha - \beta ) \ldots ..{\text{(ii) by using [tan}}(90 - \theta ){\text{ = cot}}\theta ]
{\text{From (i) and (ii),}}
{\text{Hcot}}\alpha = {\text{(H - h)cot}}(\alpha - \beta )
\Rightarrow {\text{hcot}}(\alpha - \beta ){\text{ = H[cot}}(\alpha - \beta ) - {\text{cot}}\alpha ]
\Rightarrow {\text{H = }}\dfrac{{{\text{hcot}}(\alpha - \beta )}}{{{\text{cot}}(\alpha - \beta ) - {\text{cot}}\alpha }}\;\;\;.....\;{\text{[Proved]}}
{\textbf{Hence the correct answer is option B}}
A person , standing on the bank of a river observer that the angle subtended by tree on the opposite bank is, when he retreats 40 meters from the bank, he finds the angle to be . the height of the tree and the breadth of the river are :
Please disable the adBlock and continue. Thank you.