Internal
problem
ID
[1587] Book
:
Elementary
differential
equations
with
boundary
value
problems.
William
F.
Trench.
Brooks/Cole
2001 Section
:
Chapter
2,
First
order
equations.
separable
equations.
Section
2.2
Page
52 Problem
number
:
11 Date
solved
:
Thursday, October 10, 2024 at 03:56:57 PM CAS
classification
:
[_separable]
Solve
\begin{align*} y^{\prime }&=\frac {x^{2}+3 x +2}{y-2} \end{align*}
With initial conditions
\begin{align*} y \left (1\right )&=4 \end{align*}
3.10.1 Existence and uniqueness analysis
This is non linear first order ODE. In canonical form it is written as
We assume there exists a function \(\phi \left ( x,y\right ) =c\) where \(c\) is constant, that
satisfies the ode. Taking derivative of \(\phi \) w.r.t. \(x\) gives
But since \(\frac {\partial ^{2}\phi }{\partial x\partial y}=\frac {\partial ^{2}\phi }{\partial y\partial x}\) then for the above to be valid, we require that
If the above condition is satisfied,
then the original ode is called exact. We still need to determine \(\phi \left ( x,y\right ) \) but at least we know
now that we can do that since the condition \(\frac {\partial ^{2}\phi }{\partial x\partial y}=\frac {\partial ^{2}\phi }{\partial y\partial x}\) is satisfied. If this condition is not
satisfied then this method will not work and we have to now look for an integrating
factor to force this condition, which might or might not exist. The first step is
to write the ODE in standard form to check for exactness, which is
Since \(\frac {\partial M}{\partial y}= \frac {\partial N}{\partial x}\), then the ODE is exact The following equations are now set up to solve for the
function \(\phi \left (x,y\right )\)
Where \(f(y)\) is used for the constant of integration since \(\phi \) is a function
of both \(x\) and \(y\). Taking derivative of equation (3) w.r.t \(y\) gives
But since \(\phi \) itself is a constant function, then let \(\phi =c_2\) where \(c_2\) is new
constant and combining \(c_1\) and \(c_2\) constants into the constant \(c_1\) gives the solution as
\[
c_1 = -\frac {x \left (x^{2}+\frac {9}{2} x +6\right )}{3}+\frac {y^{2}}{2}-2 y
\]
Solving for the constant of integration from initial conditions, the solution becomes
\begin{align*} -\frac {x \left (x^{2}+\frac {9}{2} x +6\right )}{3}+\frac {y^{2}}{2}-2 y = -{\frac {23}{6}} \end{align*}
`Methodsfor first order ODEs:---Trying classification methods ---tryinga quadraturetrying1st order lineartryingBernoullitryingseparable<-separable successful`