Kansas State University

Mathematics Department

Textbook Contents

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Exact Equations

Additional Examples

Solve the following initial value problem, $$\begin{align} \frac{dy}{dx} &= \frac{8y^2 - 8y + 1}{-16xy + 8x + 38y - 14} \\ y(1) &= -2 \end{align}$$ 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. $$ (-8y^2 + 8y - 1) dx + (-16xy + 8x + 38y - 14) dy = 0 $$
  2. We test for exactness. $$\frac{\partial}{\partial y}\left(-8y^2 + 8y - 1\right) = -16y + 8 = \frac{\partial}{\partial x}\left(-16xy + 8x + 38y - 14\right) $$ so the equation is exact.

  3. Write the partial differential equations. $$ \begin{align} \frac{\partial F}{\partial x} &= -8y^2 + 8y - 1\\ \frac{\partial F}{\partial y} &= -16xy + 8x + 38y - 14 \end{align}$$
  4. Integrate the first partial differential equation. $$ F(x,y) = \int (-8y^2 + 8y - 1)\,\partial x = -8xy^2 + 8xy - x + C(y) $$
  5. Integrate the second partial differential equation. $$ F(x,y) = \int (-16xy + 8x + 38y - 14)\,\partial y = -8xy^2 + 8xy + 19y^2 - 14y + \tilde{C}(x) $$
  6. Equate the expressions for F(x,y).

    Matching the expressions up, we find $C(y) = 19y^2 - 14y$ and $ \tilde{C}(x) = -x. $ So $$ F(x,y) = -8xy^2 + 8xy + 19y^2 - x - 14y. $$

  7. The solution is $F(x,y) = K.$ $$ -8xy^2 + 8xy + 19y^2 - x - 14y = K $$
Now we plug in the initial values $x = 1$ and $y = -2$ and solve for $K = 55$. So the solution to the initial value problem is $$ -8xy^2 + 8xy + 19y^2 - x - 14y = 55 $$ You may reload this page to generate additional examples.

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