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Q.

From a point on the horizontal plane, the elevation of the top of a hill is 45  . After walking 500m in towards its summit up a slope inclined at an angle of  15   to the horizon, the elevation is  75  , the height of the hill is:


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a

90m  

b

48m  

c

250 6 m  

d

72m   

answer is C.

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Detailed Solution

Let the diagram of the situation be,
seoIn ΔFDE  , we have
sinθ= Perpendicular Hypotenuse  
sin 15 = FD EF  … (1)
We know that sin 15 =sin( 45 30 )  , and sin(AB)=sinAcosBcosAsinB  .
sin 15 =sin 45 cos 30 cos 45 sin 30 = 1 2 3 2 1 2 1 2 = 3 1 2 2  
Then, (1) becomes,
3 1 2 2 = FD EF  
Given that EF=500m  , and we have
3 1 2 2 = FD EF 3 1 2 2 = FD 500 FD= 3 1 2 2 ×500  
FD=250 3 1 2 m  … (2)
In ΔFDE  , we have
cosθ= Base Hypotenuse cos 15 = ED EF  
Similarly, we get cos 15 = 3 +1 2 2  .
ED EF = 3 +1 2 2  
Given that EF=500m  , and we have
ED 500 = 3 +1 2 2 ED= 3 +1 2 2 ×500  
ED=250 3 +1 2 m  … (3)
In ΔACE  , we have
tanθ= Perpendicular Base tan 45 = AC EC 1= AC EC  
AC=EC  … (4)
In ΔABF  , we have
tan 75 = AB BF  
By using the formula tan(A+B)= tanA+tanB 1tanAtanB  , we get tan 75 =2+ 3  . So,
Question ImageQuestion ImageHence, the correct option is 3.
 
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