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Implicit Differentiation of Cylinder NOT given radius?

  • Thread starter banfill_89
  • Start date
47
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Implicit Differentiation of Cylinder NOT given radius????

1. Homework Statement

Question: Digging in his backyard, Dennis accidentally breaks a pipe attached to his water sprinkling system. water bubbles up at a rate of 1 cm^3/s, forming a circular pond of depth 0.5cm in his yard. How quickly is the surface area of the pond covering his lawn?

Given: dV/dT= 1
depth= 0.5cm

RTF: dSA/Dt

2. Homework Equations

V= pi&r^2&h
SA= 2pi&r&h + 2pi&r^2

3. The Attempt at a Solution

i attempted alot of things...i just always end up with the same problem: i dont know what r is or i cant find a way to relate r to anything.
1. Homework Statement



2. Homework Equations



3. The Attempt at a Solution
 

Answers and Replies

1,631
4
you don't actually need r at all.

[tex] V=Sh[/tex] this is the formula for calculating the volume of a cylindrical shape right? S- surface area, h- depth.
Use this info to find [tex] \frac{dS}{dt}[/tex] using the info you were given.
 
tiny-tim
Science Advisor
Homework Helper
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Welcome to PF!

2. Homework Equations

V= pi&r^2&h
SA= 2pi&r&h + 2pi&r^2
Hi banfill_89! Welcome to PF! :smile:

First, you've missed out one relevant equation … what is V in terms of t?

And your SA equation is wrong … you're asked how fast the lawn is being covered. so you don't need the sides of the cylinder. And it's πr^2, not 2πr^2.
i attempted alot of things...i just always end up with the same problem: i dont know what r is or i cant find a way to relate r to anything.
You have two equations for r … so you solve for r in the V equation (that is, you put "r =" on the left), and then you substitute that value of r into the SA equation.

You now have an SA equation with t but no r! :smile:
 
47
0
thanks

agh thanks alot guys. i was heading in that direction too but my surface area equation was screwing me up. thanks for puttin me in the right direction
 

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