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Question

When the sun's altitude changes from 30 to 60, the length of the shadow of a tower decreases by 70 m. What is the height of the tower?

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Solution


Let AD be the tower, BD be the initial shadow and CD be the final shadow.
Given that BC=70 m, ABD=30 and ACD=60
Let CD = x and AD = h.
From the right ΔCDA,
tan60=ADCD
3=hx (1)
From the right ΔBDA,
tan30=ADBD
13=h70+x (2)
Dividing (1) by (2)
313=hxh70+x
3=70+xx
2x=70
x=35
Substituting this value of x in (1), we have:-
3=h35
h=353=35×1.73=60.5560.6 m

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