A man of height 2 metres walks at a uniform speed of 5 km/hr away from a lamp post which is 6 metres high.Please Find the rate at which the length of his shadow increases.

let the length of the shadow be DE= y m .

and let the distance between the man and the lamppost be x m.

therefore dx/dt = 5km/hr

let AB be the height of lamppost and CD be the height of man. therefore

AB= 6 m and CD = 2m 

by similar triangles,

now differentiating wrt time t.

hope this helps you.

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