# Difference between revisions of "Exact differential equation"

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Suppose you are given an equation of the form: | Suppose you are given an equation of the form: | ||

− | :<math>M(t,y) + N(t,y)y' = 0\,</math> | + | :<math>M(t,y) + N(t,y)y' = 0\,</math> or <math>M(t,y) dt + N(t,y) dy = 0\,</math> |

− | + | ||

− | or | + | |

− | + | ||

− | + | ||

To find the solution of this equation, we assume that the solution is φ = constant. This means that: | To find the solution of this equation, we assume that the solution is φ = constant. This means that: |

## Revision as of 13:40, 2 August 2010

An **exact differential equation** is a differential equation that can be solved in the following manner.

Suppose you are given an equation of the form:

- or

To find the solution of this equation, we assume that the solution is φ = constant. This means that: and

φ is found by integrating M and N:

Go through the example to find φ by integrating, then check that

and

and that any function φ = some constant, when turned into the corresponding dy/dt, satisfies the original equation. Be sure to emphasize that one must check first that

(That's the condition for "exactness" of the differential form M dt + N dy.)

where .

To find , first set and . Then manipulate to get and . Integrate both sides, compare the results for , and combine the terms into one equation (for terms that show up in both expressions, only write once in the combined expression.) To solve the expression for , plug into the quadratic formula.