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Second-Order Linear Homogeneous Equations

Additional Examples

Solve the initial value problem $$ \begin{align} \frac{d^2y}{dx^2} - 18\frac{dy}{dx} + 117y &= 0 \\ y(0) &= -8 \\ y'(0) &= -10 \end{align} $$

This is a second-order linear constant-coefficient initial value problem. First we find the general solution.

Step 1: Write the equation in operator form. $$ (D^2 - 18D + 117)y = 0 $$ Step 2: Find the roots. We use the quadratic formula to compute the roots, $ 9\pm 6i $

Step 3: Write the general solution.

$$ y(x) = c_1\exp(9x)\cos(6x) + c_2\exp(9x)\sin(6x) $$ Now that we have the general solution, we plug in our initial values to find the solution to the initial value problem. First we compute, $ y'(x) = 9c_1\exp(9x)\cos(6x) - 6c_1\exp(9x)\sin(6x) + 9c_2\exp(9x)\sin(6x) + 6c_2\exp(9x)\cos(6x).$ Then we plug in $ x=0 $ to get the following equations. $$ \begin{align} y(0) = c_1 &= -8 \\ y'(0) = 9c_1 + 6c_2 &= -10 \end{align} $$ We solve these equations to get $ c_1= -8 $ and $ c_2 = 31/3.$ Finally, we plug our values for $ c_1$ and $ c_2$ into the general solution to find our solution to the initial value problem.

$$ y(x) = -8\exp(9x)\cos(6x) + (31/3)\exp(9x)\sin(6x) $$ You may reload this page to generate additional examples.


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