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# C4 setting up differential equations. Watch

1. Hey there guys. I'm wondering exactly how I'd phrase the answer to a textbook (Edexcel) question. It's a wordy one, so it's probably best if I just copy it word for word.

"Fluid flows out of a cylindrical tank with constant cross section. At time t minutes, t > 0, the volume of fluid remaining in the tank is V m^3. The rate at which the fluid flows in m^3min^-1 is proportional to the square root of V. Show that the depth h metres of fluid in the tank satisfies the differential equation

dh/dt = -k(sqrt)h, where k is a positive constant."

A note above the question implies that it involves "connected rates of change".

2. So you are told that

And you know that

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