Kansas State University

Mathematics Department

Textbook Contents

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Exact Equations

Additional Examples

Solve the following initial value problem, $$\begin{align} \frac{dy}{dx} &= \frac{y^2 + 4y + 4x - 4}{-2xy - 4x - 10y - 17} \\ y(-1) &= -3 \end{align}$$ 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. $$ (-y^2 - 4y - 4x + 4) dx + (-2xy - 4x - 10y - 17) dy = 0 $$
  2. We test for exactness. $$\frac{\partial}{\partial y}\left(-y^2 - 4y - 4x + 4\right) = -2y - 4 = \frac{\partial}{\partial x}\left(-2xy - 4x - 10y - 17\right) $$ so the equation is exact.

  3. Write the partial differential equations. $$ \begin{align} \frac{\partial F}{\partial x} &= -y^2 - 4y - 4x + 4\\ \frac{\partial F}{\partial y} &= -2xy - 4x - 10y - 17 \end{align}$$
  4. Integrate the first partial differential equation. $$ F(x,y) = \int (-y^2 - 4y - 4x + 4)\,\partial x = -xy^2 - 4xy - 2x^2 + 4x + C(y) $$
  5. Integrate the second partial differential equation. $$ F(x,y) = \int (-2xy - 4x - 10y - 17)\,\partial y = -xy^2 - 4xy - 5y^2 - 17y + \tilde{C}(x) $$
  6. Equate the expressions for F(x,y).

    Matching the expressions up, we find $C(y) = -5y^2 - 17y$ and $ \tilde{C}(x) = -2x^2 + 4x. $ So $$ F(x,y) = -xy^2 - 4xy - 2x^2 - 5y^2 + 4x - 17y. $$

  7. The solution is $F(x,y) = K.$ $$ -xy^2 - 4xy - 2x^2 - 5y^2 + 4x - 17y = K $$
Now we plug in the initial values $x = -1$ and $y = -3$ and solve for $K = -3$. So the solution to the initial value problem is $$ -xy^2 - 4xy - 2x^2 - 5y^2 + 4x - 17y = -3 $$ You may reload this page to generate additional examples.

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