# Calculus help: Derivate problem...

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.strike that.

#### vtqanh

##### Diamond Member
Surface area, (ignoring the surface at the end of the cable) S=2*pi*radius*height or S=2*pi*r*h
dS/dt = 2*pi**h*dr/dt
dr/dt = (dS/dt)/(2*pi*h) = 750/(2*3.14*1200) (100 ft = 1200 inches) ~ 0.0995 in^2/yr
That's the radius, the diameter is gonna increase two times as fast, thus, 0.199

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#### yoyofatjo

##### Banned
Originally posted by: fyleow
Originally posted by: vtqanh
Surface area, (ignoring the surface at the end of the cable) S=2*pi*radius*height or S=2*pi*r*h
dS/dt = 2*pi**h*dr/dt
dr/dt = (dS/dt)/(2*pi*h) = 750/(2*3.14*1200) (100 ft = 1200 inches) ~ 0.0995 in^2/yr
That's the radius, the diameter is gonna increase two times as fast, thus, 0.199

Ok, that's right. Have time for another question? (Practice test, not homework btw)

Is the test open-note, open-book, open-your-web-browser-and-login-to-ATForums?

#### dopcombo

##### Golden Member
MUahahahahaha..

Yeah right its not homework.

If you just read your textbook, you would have known how to do that qn. Very elementary calculus.

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#### vtqanh

##### Diamond Member
Originally posted by: fyleow
Other problem here

BTW, thanks for the help with the first one vtqanh

The second one you need to draw it out on a xyz plane, it will be a lot easier. The car starts at the x -axis at (20,0,0) - since the bridge is 20 feet high - and is moving along the y axis. The boat starts at (0,0,0) and is moving along the z-axis. Just find out their coordinates 10 seconds later with the given velocity, then you will be able to calculate the distance. Also note the differences in units (foot and mile)

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