Calculus related rates?

Question

A street light is at the top of a 13 ft tall pole. A woman 6 ft tall walks away from the pole with a speed of 5 ft/sec along a straight path. How fast is the tip of her shadow moving when she is 30 ft from the base of the pole?

Answer 1

you need to find d(x+y)/dtwhen x=30
you similar triangles 13/6=(x+y)/y=> 13y=6x+6y=>7y=6x
y=6x/7
so d(x+6x/7)/dt=d(13x/7)/dt
=13/7dx/dt=13/7*5=65/7 ft/sec that is answer
but i want you to make sure you copy the number is right
i think that should be 15 ft tall pole. that is easy to find but anyway
the answer for this question is 65/7
sorry for any convenience cuz i cant draw the picture to you.
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: 13 ......... ;
: ................ 6 ;
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x ......................... .......... y