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Maths c4 differentiation

A boy 1.5 m tall runs directly away from a light source which is fixed 2m above a horizontal road. If he runs at a speed of 3ms^-1 then find rate at which his shadow is lengthening.

I did dv/dt is 3. I guess u could do dv/dt =dv/dx times dx/dt but I can't get any further...

Thanks! - Jay

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Reply 1

wofldog

Original post by Jaydude

First of all, try finding an expression for

l

(length of his shadow) in terms of

x

(displacement from the bottom of the lamppost).

This expertly drawn diagram might give you some help.

Reply 2

Jaydude

Thank you, solved it; so you get tan theta in terms of X and L for two triangles and equate them. You get x in terms of l , so on.

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