5.1.313 AP Calculus Exam DE on bird weight

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Discussion Overview

The discussion revolves around a differential equation related to bird weight, specifically analyzing the growth dynamics of the bird's weight over time. Participants explore the mathematical modeling of weight gain, including the implications of the derived equations and the behavior of the function representing weight.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • Some participants note that if $B=0$, there is no weight gain, while others discuss the implications of $\dfrac{dB}{dt} > 0$ for $20 \le B < 100$ regarding the change in the bird's weight.
  • One participant presents the second derivative $\dfrac{d^2B}{dt^2} = -\dfrac{1}{5}(100-B)$, concluding that $B(t)$ is concave down everywhere.
  • Another participant challenges the differentiation process, stating that the right side was not differentiated correctly and suggests the left side should involve $dB$ instead of $dt$.
  • Participants derive the function $B(t) = 100 - 80e^{-t/5}$, describing it as an example of inhibited exponential growth, with a limit of $100$ as $t$ approaches infinity.
  • There is a mention of using a graphing program, with some participants sharing tools they have used for graphing the function.

Areas of Agreement / Disagreement

Participants express disagreement regarding the differentiation steps and the correct formulation of the equations. There is no consensus on the correct approach to the differentiation process, and multiple interpretations of the mathematical steps are presented.

Contextual Notes

Some participants highlight missing assumptions in the differentiation process and the need for clarity on the application of the differential equation. The discussion includes unresolved mathematical steps and varying interpretations of the equations involved.

karush
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I just posted a image due to overleaf newcommands and graph

ok (a) if we use f(20) then the $B=0$ so their no weight gain.

(b), (c), was a little baffled and not sure how this graph was derived...
 

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(a) $\dfrac{dB}{dt} > 0$ for $20 \le B < 100$ ... what does that say about the change in the bird's weight?

(b) $\dfrac{d}{dt} \left[\dfrac{dB}{dt} = \dfrac{1}{5}(100-B) \right]$

$\dfrac{d^2B}{dt^2} = -\dfrac{1}{5}(100-B) < 0 \implies B(t) \text{ is concave down everywhere}$

(c) $\dfrac{-1}{100-B} \, dt = -\dfrac{1}{5} \, dt$

$\log|100-B| = -\dfrac{t}{5} + C$

$B(0) = 20 \implies C = \log(80)$

$100-B = 80e^{-t/5} \implies B = 100 - 80e^{-t/5}$

the function $B(t)$ is an example of inhibited exponential growth ... note $$\lim_{t \to \infty} B(t) = 100$$
 

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did you do the graph in tikx?

great help... appreciate all the steps
 
karush said:
did you do the graph in tikx?

no, I've had this free graphing program for quite a while ...

https://www.padowan.dk/
 
skeeter said:
(a) $\dfrac{dB}{dt} > 0$ for $20 \le B < 100$ ... what does that say about the change in the bird's weight?

(b) $\dfrac{d}{dt} \left[\dfrac{dB}{dt} = \dfrac{1}{5}(100-B) \right]$
$\dfrac{d^2B}{dt^2} = -\dfrac{1}{5}(100-B) < 0 \implies B(t) \text{ is concave down everywhere}$
No, you didn't differentiate on the right side.
$\frac{d^2B}{dt^2}= \dfrac{d}{dt}[dfrac{1}{5}(100- B)= \dfrac{1}{5}(-B)=- \frac{1}{5}B$

(c) $\dfrac{-1}{100-B} \, dt = -\dfrac{1}{5} \, dt$
The left side should be $\dfrac{-1}{100- B}dB$. not "dt".

$\log|100-B| = -\dfrac{t}{5} + C$

$B(0) = 20 \implies C = \log(80)$

$100-B = 80e^{-t/5} \implies B = 100 - 80e^{-t/5}$

the function $B(t)$ is an example of inhibited exponential growth ... note $$\lim_{t \to \infty} B(t) = 100$$
 
skeeter said:
no, I've had this free graphing program for quite a while ...

https://www.padowan.dk/
looks pretty clean and basic... which is very nice..
 
HallsofIvy said:
No, you didn't differentiate on the right side.
$\frac{d^2B}{dt^2}= \dfrac{d}{dt}[dfrac{1}{5}(100- B)= \dfrac{1}{5}(-B)=- \frac{1}{5}B$The left side should be $\dfrac{-1}{100- B}dB$. not "dt".

yep ... it happens.
 

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