Complex Exponential and Logarithmic Functions Word Problem

Blistex

New member
Joined
Jul 27, 2015
Messages
3
Hey,
So the question below is a question from my maths assignment that basically has the whole grade stumped so any help or ideas will be GREATLY appreciated.
I have been given the values for A,B and C. Where A = 90, B = 0.96 and C = 1100

A typical fish farming process might involve placing small fish (fingerlings) in a dam, waiting for them to grow, and then harvesting all of the remaining fish.

People who rely on growing fish as a source of food are faced with the problem of working out when is the best time to harvest the fish, because although the fish grow bigger as time passes, unfortunately some of them die before they are harvested for food.

Data was collected and analysed for a particular species of fish. The following information was obtained:

Fish Length
The following formula gives a relationship between the fish length (L) in centimetres and time elapsed (t) in months since the fingerlings were placed in the dam:
L = A (1 – Bt )


Fish Weight
The following weight-length data is available for the species concerned: (It was also recommended that we make an equation out of this table and the equation was as follows, W = 0.0141L3.0164)

Length (cm)10.125.032.635.443.845.555.7
Weight (g)15236520660125014252590


Length of Life
For every ‘C’ fingerlings of this species of fish placed in the dam, the number, n, still alive after t months is given by the formula:
N = C x Bt


Based on this information, find the best time to harvest all of the fish remaining in the dam in order to get the maximum weight of live fish.

Thanks for taking time to read and maybe even attempt the questions and any progress Is greatly appreciated.
 
Last edited by a moderator:
Hey,
So the question below is a question from my maths assignment that basically has the whole grade stumped so any help or ideas will be GREATLY appreciated.
I have been given the values for A,B and C. Where A = 90, B = 0.96 and C = 1100

A typical fish farming process might involve placing small fish (fingerlings) in a dam, waiting for them to grow, and then harvesting all of the remaining fish.

People who rely on growing fish as a source of food are faced with the problem of working out when is the best time to harvest the fish, because although the fish grow bigger as time passes, unfortunately some of them die before they are harvested for food.

Data was collected and analysed for a particular species of fish. The following information was obtained:

Fish Length
The following formula gives a relationship between the fish length (L) in centimetres and time elapsed (t) in months since the fingerlings were placed in the dam:
L = A (1 – Bt )


Fish Weight
The following weight-length data is available for the species concerned: (It was also recommended that we make an equation out of this table and the equation was as follows, W = 0.0141L3.0164)

Length (cm)10.125.032.635.443.845.555.7
Weight (g)15236520660125014252590

Length of Life
For every ‘C’ fingerlings of this species of fish placed in the dam, the number, n, still alive after t months is given by the formula:
N = C x Bt


Based on this information, find the best time to harvest all of the fish remaining in the dam in order to get the maximum weight of live fish.

Thanks for taking time to read and maybe even attempt the questions and any progress Is greatly appreciated.

At a time t you will have N fish weighing W grams for a total of
T = N * W
fish. To find the maximum for T at time t, take the derivative of T and set it to zero
T' = N' W + N W' = C ln(B) Bt W + ...Can you take it from there?
 
Update:
I have derived the T equation but I am stuck at a step where I have the L still in the equation, so how do I remove the L to be able to find the maximum of t

T' = (46.7841L^2.0104 - 0.633149L^3.0164)*(0.96^t)
 
Update:
I have derived the T equation but I am stuck at a step where I have the L still in the equation, so how do I remove the L to be able to find the maximum of t

T' = (46.7841L^2.0104 - 0.633149L^3.0164)*(0.96^t)

The form appears correct but the numbers don't seem to work out
T = W N = 0.0141 L3.0164 C Bt = 0.0141 A3.0164 (1-Bt)3.0164 C Bt
= 0.0141 903.0164 (1-0.96t)3.0164 1100 Bt
= 0.0141 903.0164 (1-0.96t)3.0164 1100 0.96t
~12172756 (1-0.96t)3.0164 0.96t
or, in terms of how you have written it
T = 15.51 L3.0164 0.96t

So, it appears that, in your answer, you have taken into account the ln(B) for the - 0.633149L^3.0164 part of your equation but you have not taken it into account for the 46.7841L^2.0104 part of the equation. Remember, I said you would need the chain rule and
dL3.0164/dt = 3.0164 L2.0164 dL/dt
You dropped the dL/dt
 
Update:
I have derived the T equation but I am stuck at a step where I have the L still in the equation, so how do I remove the L to be able to find the maximum of t

T' = (46.7841L^2.0104 - 0.633149L^3.0164)*(0.96^t)

To answer your equation, you don't remove the L to start with. You have
T' = (a L2.0164 - b L3.0164) 0.96t
Since 0.96t is never zero, for T' to be zero, we have
a L2.0164 - b L3.0164 = L2.0164 (a - b L) = 0
or either L=0 or L=a/b = A (1-Bt)
 
Top