A man standing 3 feet from the base of a lamppost casts a shadow 4 feet long. If the man is 6 feet tall and walks away from the lamppost at a speed of 400 feet per minute, at what rate will his shadow lengthen? How fast is the tip of his shadow moving?(adsbygoogle = window.adsbygoogle || []).push({});

I'm unsure of how to solve the 2nd part, a bump would be good. Kinda brain dead atm :)

Here's the first part: Just use similar triangles

[tex]\frac{z}{x+y}=\frac 6 y \ \ \ z=\frac{21}{2}ft[/tex]

[tex]y=x\left(\frac{6}{z-6}\right)[/tex]

[tex]\frac{dy}{dt}=\frac{dx}{dt}\left(\frac{6}{z-6}\right)=\frac{1600}{3}\frac{ft}{min}[/tex]

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# Homework Help: Related Rates street light shadow

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