10. (a) Write \(\frac { 1 } { ( H - 5 ) ( H + 3 ) }\) in partial fraction form.
The depth of water in a storage tank is being monitored.
The depth of water in the tank, \(H\) metres, is modelled by the differential equation
$$\frac { \mathrm { d } H } { \mathrm {~d} t } = - \frac { ( H - 5 ) ( H + 3 ) } { 40 }$$
where \(t\) is the time, in days, from when monitoring began.
Given that the initial depth of water in the tank was 13 m ,
(b) solve the differential equation to show that
$$H = \frac { 10 + 3 \mathrm { e } ^ { - 0.2 t } } { 2 - \mathrm { e } ^ { - 0.2 t } }$$
(c) Hence find the time taken for the depth of water in the tank to fall to 8 m .
(Solutions relying entirely on calculator technology are not acceptable.)
According to the model, the depth of water in the tank will eventually fall to \(k\) metres.
(d) State the value of the constant \(k\).
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