AP Calculus AB : Solving separable differential equations and using them in modeling

Study concepts, example questions & explanations for AP Calculus AB

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Example Questions

Example Question #21 : Solving Separable Differential Equations And Using Them In Modeling

Solve the separable differential equation:

\displaystyle \frac{\mathrm{d} y}{\mathrm{d} x}=y\cos(x)

given the condition at

\displaystyle x=\pi, y=2

Possible Answers:

\displaystyle y=e^{\sin(x)}

\displaystyle y=x

\displaystyle y=Ce^{\sin(x)}

\displaystyle y=2e^{\sin(x)}

Correct answer:

\displaystyle y=2e^{\sin(x)}

Explanation:

To solve the separable differential equation, we must separate x and y to the same sides as their respective derivatives:

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

Next, we integrate both sides:

\displaystyle \int \frac{dy}{y}=\ln\left | y\right |+C\displaystyle \int \cos(x)dx=\sin(x)+C

The integrals were solved using the identical rules.

The two constants of integration are now combined to make a single one:

\displaystyle \ln \left | y\right |= \sin(x)+C

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

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

Finally, we solve for the integration constant using the given condition:

\displaystyle 2=Ce^{\sin(\pi)}=Ce^0=C

Our final answer is

\displaystyle y=2e^{\sin(x)}

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