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# Edexcel C4 Rate of change modelling question watch

1. I'm stuck on the following question (7a):

I have to make a rate of change equation for it, I don't understand all this information it is giving me. My working is like this:
dx/dt is proprotional to x(1-x)
dx/dt = -kx(1-x)

On the mark scheme it has a positive k instead of a negative k, i don't know why since the crop is going down so it's decaying, either way I don't know how to go from here. I have no idea what to do next, it is talking about 2 different things, a plague of locusts discovering crop at x=1/4, and the crop depleting totally at t=6. I don't understand why the crop is been destroyed before the plague of locusts discover it then destroy it even more.

Any insight on this question would be greatly appreciated thanks
2. See if this link helps

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3. (Original post by Sayless)
I'm stuck on the following question (7a):

I have to make a rate of change equation for it, I don't understand all this information it is giving me. My working is like this:
dx/dt is proprotional to x(1-x)
dx/dt = -kx(1-x)

On the mark scheme it has a positive k instead of a negative k, i don't know why since the crop is going down so it's decaying, either way I don't know how to go from here. I have no idea what to do next, it is talking about 2 different things, a plague of locusts discovering crop at x=1/4, and the crop depleting totally at t=6. I don't understand why the crop is been destroyed before the plague of locusts discover it then destroy it even more.

Any insight on this question would be greatly appreciated thanks
As for your first question, is the proportion of crop destroyed so it starts at 0 (no crop destroyed) and grows to 1 (all crop destroyed) over time, hence its rate of change must be positive, else it couldn't increase. Since we have then we have so if your is negative, then you will end up with a negative rate of change.

As for your second question, I'm afraid that I don't understand it.
4. Thanks for the help

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