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A certain substance is formed in a chemical reaction. The mass of substance formed $t$ seconds after the start of the reaction is $x$ grams. At any time the rate of formation of the substance is proportional $\left( {20 - x} \right)$. When $t = 0$, $x = 0$ and $\frac{{dx}}{{dt}} = 1$.

a) Show that $x$ and $t$ satisfy the differential equation

$\frac{{dx}}{{dt}} = 0.05\left( {20 - x} \right)$.

b) Find, in any form, the solution of this differential equation.

c) Find $x$ when $t = 10$, giving your answer correct to 1 decimal place.

d) State what happens to the value of $x$ as $t$ becomes very large.

پاسخ تشریحی :
نمایش پاسخ

a) State or imply $\frac{{dx}}{{dt}} = k\left( {20 - x} \right)$

Show that $k = 0.05$

b) Separate variables correctly and integrate both sides

Obtain term $ - ln\left( {20 - x} \right)$, or equivalent

Obtain term $\frac{1}{{20}}t$, or equivalent

Evaluate a constant or use limits $t = 0$, $x = 0$ in a solution containing terms $\alpha {\text{ }}ln\left( {20 - x} \right)$ and $bt$

Obtain correct answer in any form, e.g. $ln{\text{ }}20 - ln\left( {20 - x} \right) = \frac{1}{{20}}t$

c) Substitute $t = 10$ and calculate $x$

Obtain answer $x = 7.9$

d) State that $x$ approaches 20

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