AP Calculus BC : Modeling by Solving Separable Differential Equations

Study concepts, example questions & explanations for AP Calculus BC

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Example Questions

Example Question #191 : Ap Calculus Bc

Solve the separable differential equation

\(\displaystyle \frac{\mathrm{d} y}{\mathrm{d} x}= yx^2\)

with the condition \(\displaystyle y(0)=4\).

Possible Answers:

\(\displaystyle y=Ce^{\frac{x^2}{2}}\)

\(\displaystyle y=4e^{\frac{x^2}{3}}\)

\(\displaystyle y=4e^{\frac{x^3}{3}}\)

\(\displaystyle y=Ce^{\frac{64}{3}}\)

\(\displaystyle y=Ce^{\frac{x^3}{3}}\)

Correct answer:

\(\displaystyle y=4e^{\frac{x^3}{3}}\)

Explanation:

To solve the separable differential equation, we must separate x and y, dx and dy respectively to opposite sides:

\(\displaystyle \frac{dy}{y}=x^2dx\)

Integrating both sides, we get

\(\displaystyle \ln\left | y\right |=\frac{ x^3}{3}+C\)

The rules of integration used were

\(\displaystyle \int \frac{dx}{x}=\ln\left | x\right |+C\)\(\displaystyle \int x^ndx= \frac{x^{n+1}}{n+1}+C\)

The constants of integration merged into one.

Now, we exponentiate both sides of the equation to solve for y, and use the properties of exponents to simplify:

\(\displaystyle y=e^{\frac{x^3}{3}}+e^C=e^{{\frac{x^3}{3}}\cdot C}=Ce^{\frac{x^3}{3}}\)

To solve for C, we use our given condition:

\(\displaystyle 4=Ce^0\)

\(\displaystyle 4=C\)

Our final answer is

\(\displaystyle y=4e^{\frac{x^3}{3}}\)

 

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