Kansas State University

Mathematics Department

Textbook Contents

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Exact Equations

Additional Examples

Solve the following initial value problem, $$\begin{align} \frac{dy}{dx} &= \frac{10x - 6y - 20}{6x + 2y + 4} \\ y(-2) &= -4 \end{align}$$ This can be written as an exact equation. First we find the general solution following the paradigm.

  1. We write the equation in the standard form, M dx + N dy = 0. $$ (-10x + 6y + 20) dx + (6x + 2y + 4) dy = 0 $$
  2. We test for exactness. $$\frac{\partial}{\partial y}\left(-10x + 6y + 20\right) = 6 = \frac{\partial}{\partial x}\left(6x + 2y + 4\right) $$ so the equation is exact.

  3. Write the partial differential equations. $$ \begin{align} \frac{\partial F}{\partial x} &= -10x + 6y + 20\\ \frac{\partial F}{\partial y} &= 6x + 2y + 4 \end{align}$$
  4. Integrate the first partial differential equation. $$ F(x,y) = \int (-10x + 6y + 20)\,\partial x = -5x^2 + 6xy + 20x + C(y) $$
  5. Integrate the second partial differential equation. $$ F(x,y) = \int (6x + 2y + 4)\,\partial y = 6xy + y^2 + 4y + \tilde{C}(x) $$
  6. Equate the expressions for F(x,y).

    Matching the expressions up, we find $C(y) = y^2 + 4y$ and $ \tilde{C}(x) = -5x^2 + 20x. $ So $$ F(x,y) = -5x^2 + 6xy + y^2 + 20x + 4y. $$

  7. The solution is $F(x,y) = K.$ $$ -5x^2 + 6xy + y^2 + 20x + 4y = K $$
Now we plug in the initial values $x = -2$ and $y = -4$ and solve for $K = -12$. So the solution to the initial value problem is $$ -5x^2 + 6xy + y^2 + 20x + 4y = -12 $$ You may reload this page to generate additional examples.

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