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# Core 4 - Differentiation watch

1. I'm not sure if what I've done so far is correct but I can't figure what to do next.

Each edge of a contracting cube is decreasing at a rate of 0.06cms^-1.
Find rate of decrease of the volume.

Here's what Ive done:
dv/dt = dx/dt x dv/dx

v = x^3
dv/dx = 3x^2
Sub in

dv/dt = 0.06 x 3x^2
dv/dt = 0.18x^2

Here is where I get stuck, I tried to find value for x but am not getting correct answer.
Thanks for any help.
2. You said both v and x are functions of time to you'd want v on the RHS of your last equation. I haven't done this in a long time, but i presume you have where f(v(t))= and solve as usual. substitute x back in afterwards because you know the relationship
3. Ok, thanks for your help.

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