Volume 3: The Calculus of Life

Workbook 25.3: Differential Equations

Directives for the Origin-Hunter:

1. Integrate first: Find the general formula ($+ C$).
2. Identify the Witness: Write down the $(x, y)$ point given in the problem.
3. Solve for C: Plug $x$ and $y$ into your formula and find the secret constant.
4. The Particular Path: Write your final answer with the real number for $C$.

Part I: Solving for C

Find the **Particular Solution** for each differential equation.

$y' = 4x$. The curve passes through $(1, 10)$.

General: $y = 2x^2 + C$
Solve for C: $10 = 2(1)^2 + C \implies 10 = 2 + C \implies C = ...$
Final: $y = 2x^2 + ...$

$y' = \cos x$. The curve passes through $(0, 5)$.

General: $y = \sin x + C$
Solve for C: $5 = \sin(0) + C \implies ...$

$y' = e^x$. The curve passes through $(0, 100)$.

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Part II: Particular Restoration

The Falling Arrow: The velocity of an arrow is $v(t) = -32t + 100$.
Its initial height (at $t=0$) is 6 feet.
Find the height function $h(t) = \int v(t) dt$.

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The Rebuilt Wall: The rate of building a wall is $R(x) = 3x^2 + 10$.
After 2 hours ($x=2$), the wall was 20 feet high.
How high was the wall when they started ($x=0$)?

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The Logic Check:

If you have two different people with the same speed ($y' = 10$), but one has $C=5$ and the other has $C=50$... will they ever meet? Will the distance between them ever change? Why?

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Part III: The Inverse of 1/x

Find the Particular Solution for $y' = 1/x$ if $y(1) = 0$.

General: $y = \ln|x| + C$
Solve: $0 = \ln(1) + C \implies ...$

Part IV: The Challenge (The Population Shift)

The City of Zion

The growth rate of the city is $P'(t) = 20e^t + 5$.
Initially, there were 1,000 people.
Task: Find the population formula $P(t)$ and calculate the population after 10 years.

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Part V: Transmission (The Echad Extension)

Teacher Log: The Starting Line

Objective: Explain Initial Conditions using a race.

The Activity:
1. Have two students race.
2. Give one a 5-step head start.
3. Tell them both to walk at the same slow speed.

The Lesson: "Their speed is the same ($f'$), but their position ($f$) is different because of where they started ($C$). God cares about our individual starting lines!"


Response: ___________________________________________________________

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