Applications Of Derivatives Question 217
Question: A man of height 1.8 metre is moving away from a lamp post at the rate of 1.2 $ m/\sec . $ If the height of the lamp post be 4.5 metre, then the rate at which the shadow of the man is lengthening is
[MP PET 1989]
Options:
A) $ 0.4m/\sec $
B) $ 0.8m/\sec $
C) $ 1.2,\text{m}/\sec $
D) None of these
Show Answer
Answer:
Correct Answer: B
Solution:
$ \frac{dy}{dt}=1.2. $ According to the figure $ x=\frac{2}{3}y $
therefore $ \frac{dx}{dt}=\frac{2}{3}.\frac{dy}{dt} $
$ \therefore $ Required rate of length of shadow $ \frac{dx}{dt}=0.8m/s $ .
BETA
