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A biologist is investigating the spread of a weed in a particular region. At time $t$ weeks after the start of the investigation, the area covered by the weed is $A{\text{ }}{m^2}$. The biologist claims that the rate of increase of $A$ is proportional to $\surd \left( {2A - 5} \right)$.

a) Write down a differential equation representing the biologist’s claim.

b) At the start of the investigation, the area covered by the weed was $7{\text{ }}{m^2}$ and, 10 weeks later, the area covered was $27{\text{ }}{m^2}$.  Assuming that the biologist’s claim is correct, find the area covered 20 weeks after the start of the investigation.

پاسخ تشریحی :
نمایش پاسخ

a) State $\frac{{dA}}{{dt}} = k\sqrt {2A - 5} $

b) Separate variables correctly and attempt integration of each side

Obtain ${\left( {2A - 5} \right)^{\frac{1}{2}}} = ...$ or equivalent

Obtain $ = kt$ or equivalent

Use $t = 0$ and $A = 7$ to find value of arbitrary constant

Obtain $C = 3$ or equivalent

Use $t = 10$ and $A = 27$ to find $k$

Obtain $k = 0.4$ or equivalent

Substitute $t = 20$ and values for $C$ and $k$ to find value of $A$

Obtain 63

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