Answer :

Given: man 160cm tall walks away from a source of light situated at the top of a pole 6 m high, at the rate of 1.1m/sec


To find the rate at which the length of his shadow increases when he is 1m away from the pole


Let AB be the lamp post and let MN be the man of height 160cm or 1.6m.


Let AL = l meter and MS be the shadow of the man


Let length of the shadow MS = s (as shown in the below figure)



Given man walks at the speed of 1.1 m/sec



So the rate at which the length of the man’s shadow increases will be


Consider ΔASB




Now consider ΔMSN, we get




So from equation(ii) and (iii),






Applying derivative with respect to time on both sides we get,




[from equation(i)]



Hence the rate at which the length of his shadow increases by 0.4 m/sec, and it is independent to the current distance of the man from the base of the light.


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