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### First Order Linear Equations

#### Additional Examples

Solve the following initial value problem
$$
\begin{align}
\frac{dy}{dx} + 9 y &= 3\exp(-7 x) \\
y(0) &= 7
\end{align}
$$
This is a linear equation. First we find the general solution following the paradigm.
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©2010, 2014 Andrew G. Bennett

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- Find the integrating factor $$ \mu(x) = \exp(\int 9 dx) = \exp(9x) $$
- Multiply through by the integrating factor $$ \exp(9x) \frac{dy}{dx} + 9\exp(9x)y = 3\exp(-7 x)\exp(9x) = 3\exp(2x) $$
- Recognize the left-hand-side as $\displaystyle \frac{d}{dx}(\mu(x)y).$ $$ \frac{d}{dx}(\exp(9x)y) =3\exp(2x) $$
- Integrate both sides. $$ \exp(9x)y = (3/2)exp(2x) + C $$
- Divide through by $\mu(x)$ to solve for $ y.$ $$y = (3/2)exp(-7x) + C\exp(-9x) $$

If you have any problems with this page, please contact bennett@math.ksu.edu.

©2010, 2014 Andrew G. Bennett