1.78 problem 81

Internal problem ID [3873]
Book : Differential equations for engineers by Wei-Chau XIE, Cambridge Press 2010
Section : Chapter 2. First-Order and Simple Higher-Order Differential Equations. Page 78
Problem number : 81
Date solved : Thursday, October 17, 2024 at 04:54:29 AM
CAS classification : [[_1st_order, _with_linear_symmetries], _Clairaut]

Solve

\begin{align*} y&=x y^{\prime }+{y^{\prime }}^{3} \end{align*}

1.78.1 Solved as first order Clairaut ode

Time used: 0.092 (sec)

This is Clairaut ODE. It has the form

\[ y=x y^{\prime }+g\left (y^{\prime }\right ) \]

Where \(g\) is function of \(y'(x)\). Let \(p=y^{\prime }\) the ode becomes

\begin{align*} y = p^{3}+x p \end{align*}

Solving for \(y\) from the above results in

\begin{align*} y &= p^{3}+x p\tag {1A} \end{align*}

The above ode is a Clairaut ode which is now solved.

We start by replacing \(y^{\prime }\) by \(p\) which gives

\begin{align*} y&=p^{3}+x p\\ &=p^{3}+x p \end{align*}

Writing the ode as

\begin{align*} y&= x p +g \left (p \right ) \end{align*}

We now write \(g\equiv g\left ( p\right ) \) to make notation simpler but we should always remember that \(g\) is function of \(p\) which in turn is function of \(x\). Hence the above becomes

\begin{align*} y = x p +g\tag {1} \end{align*}

Then we see that

\begin{align*} g&=p^{3} \end{align*}

Taking derivative of (1) w.r.t. \(x\) gives

\begin{align*} p &=\frac {d}{dx}\left (x p+g\right ) \\ p & =\left ( p+x\frac {dp}{dx}\right ) +\left ( g' \frac {dp}{dx}\right ) \\ p & =p+\left ( x+g'\right ) \frac {dp}{dx}\\ 0 & =\left ( x+g'\right ) \frac {dp}{dx} \end{align*}

Where \(g'\) is derivative of \(g\left ( p\right ) \) w.r.t. \(p\).

The general solution is given by

\begin{align*} \frac {dp}{dx} & =0\\ p &=c_{1} \end{align*}

Substituting this in (1) gives the general solution as

\begin{align*} y = c_1^{3}+c_1 x \end{align*}

The singular solution is found from solving for \(p\) from

\begin{align*} x+g'\left ( p\right ) &=0 \end{align*}

And substituting the result back in (1). Since we found above that \(g=p^{3}\), then the above equation becomes

\begin{align*} x+g'\left ( p\right ) &= 3 p^{2}+x\\ &= 0 \end{align*}

Solving the above for \(p\) results in

\begin{align*} p_{1} &=y = -\frac {2 \sqrt {3}\, \left (-x \right )^{{3}/{2}}}{9}\\ p_{2} &=y = \frac {2 \sqrt {3}\, \left (-x \right )^{{3}/{2}}}{9} \end{align*}

Substituting the above back in (1) results in

\begin{align*} y = -\frac {2 \sqrt {3}\, \left (-x \right )^{{3}/{2}}}{9}\\ y = \frac {2 \sqrt {3}\, \left (-x \right )^{{3}/{2}}}{9} \end{align*}

1.78.2 Maple step by step solution

\[ \begin {array}{lll} & {} & \textrm {Let's solve}\hspace {3pt} \\ {} & {} & y \left (x \right )=x \left (\frac {d}{d x}y \left (x \right )\right )+\left (\frac {d}{d x}y \left (x \right )\right )^{3} \\ \bullet & {} & \textrm {Highest derivative means the order of the ODE is}\hspace {3pt} 1 \\ {} & {} & \frac {d}{d x}y \left (x \right ) \\ \bullet & {} & \textrm {Solve for the highest derivative}\hspace {3pt} \\ {} & {} & \left [\frac {d}{d x}y \left (x \right )=\frac {\left (108 y \left (x \right )+12 \sqrt {12 x^{3}+81 y \left (x \right )^{2}}\right )^{{1}/{3}}}{6}-\frac {2 x}{\left (108 y \left (x \right )+12 \sqrt {12 x^{3}+81 y \left (x \right )^{2}}\right )^{{1}/{3}}}, \frac {d}{d x}y \left (x \right )=-\frac {\left (108 y \left (x \right )+12 \sqrt {12 x^{3}+81 y \left (x \right )^{2}}\right )^{{1}/{3}}}{12}+\frac {x}{\left (108 y \left (x \right )+12 \sqrt {12 x^{3}+81 y \left (x \right )^{2}}\right )^{{1}/{3}}}-\frac {\mathrm {I} \sqrt {3}\, \left (\frac {\left (108 y \left (x \right )+12 \sqrt {12 x^{3}+81 y \left (x \right )^{2}}\right )^{{1}/{3}}}{6}+\frac {2 x}{\left (108 y \left (x \right )+12 \sqrt {12 x^{3}+81 y \left (x \right )^{2}}\right )^{{1}/{3}}}\right )}{2}, \frac {d}{d x}y \left (x \right )=-\frac {\left (108 y \left (x \right )+12 \sqrt {12 x^{3}+81 y \left (x \right )^{2}}\right )^{{1}/{3}}}{12}+\frac {x}{\left (108 y \left (x \right )+12 \sqrt {12 x^{3}+81 y \left (x \right )^{2}}\right )^{{1}/{3}}}+\frac {\mathrm {I} \sqrt {3}\, \left (\frac {\left (108 y \left (x \right )+12 \sqrt {12 x^{3}+81 y \left (x \right )^{2}}\right )^{{1}/{3}}}{6}+\frac {2 x}{\left (108 y \left (x \right )+12 \sqrt {12 x^{3}+81 y \left (x \right )^{2}}\right )^{{1}/{3}}}\right )}{2}\right ] \\ \bullet & {} & \textrm {Solve the equation}\hspace {3pt} \frac {d}{d x}y \left (x \right )=\frac {\left (108 y \left (x \right )+12 \sqrt {12 x^{3}+81 y \left (x \right )^{2}}\right )^{{1}/{3}}}{6}-\frac {2 x}{\left (108 y \left (x \right )+12 \sqrt {12 x^{3}+81 y \left (x \right )^{2}}\right )^{{1}/{3}}} \\ \bullet & {} & \textrm {Solve the equation}\hspace {3pt} \frac {d}{d x}y \left (x \right )=-\frac {\left (108 y \left (x \right )+12 \sqrt {12 x^{3}+81 y \left (x \right )^{2}}\right )^{{1}/{3}}}{12}+\frac {x}{\left (108 y \left (x \right )+12 \sqrt {12 x^{3}+81 y \left (x \right )^{2}}\right )^{{1}/{3}}}-\frac {\mathrm {I} \sqrt {3}\, \left (\frac {\left (108 y \left (x \right )+12 \sqrt {12 x^{3}+81 y \left (x \right )^{2}}\right )^{{1}/{3}}}{6}+\frac {2 x}{\left (108 y \left (x \right )+12 \sqrt {12 x^{3}+81 y \left (x \right )^{2}}\right )^{{1}/{3}}}\right )}{2} \\ \bullet & {} & \textrm {Solve the equation}\hspace {3pt} \frac {d}{d x}y \left (x \right )=-\frac {\left (108 y \left (x \right )+12 \sqrt {12 x^{3}+81 y \left (x \right )^{2}}\right )^{{1}/{3}}}{12}+\frac {x}{\left (108 y \left (x \right )+12 \sqrt {12 x^{3}+81 y \left (x \right )^{2}}\right )^{{1}/{3}}}+\frac {\mathrm {I} \sqrt {3}\, \left (\frac {\left (108 y \left (x \right )+12 \sqrt {12 x^{3}+81 y \left (x \right )^{2}}\right )^{{1}/{3}}}{6}+\frac {2 x}{\left (108 y \left (x \right )+12 \sqrt {12 x^{3}+81 y \left (x \right )^{2}}\right )^{{1}/{3}}}\right )}{2} \\ \bullet & {} & \textrm {Set of solutions}\hspace {3pt} \\ {} & {} & \left \{\mathit {workingODE} , \mathit {workingODE} , \mathit {workingODE}\right \} \end {array} \]

1.78.3 Maple trace

`Methods for first order ODEs: 
   *** Sublevel 2 *** 
   Methods for first order ODEs: 
   -> Solving 1st order ODE of high degree, 1st attempt 
   trying 1st order WeierstrassP solution for high degree ODE 
   trying 1st order WeierstrassPPrime solution for high degree ODE 
   trying 1st order JacobiSN solution for high degree ODE 
   trying 1st order ODE linearizable_by_differentiation 
   trying differential order: 1; missing variables 
   trying dAlembert 
   <- dAlembert successful`
 

1.78.4 Maple dsolve solution

Solving time : 0.086 (sec)
Leaf size : 37

dsolve(y(x) = x*diff(y(x),x)+diff(y(x),x)^3, 
       y(x),singsol=all)
 

\begin{align*} y \left (x \right ) &= \frac {2 \sqrt {3}\, \left (-x \right )^{{3}/{2}}}{9} \\ y \left (x \right ) &= -\frac {2 \sqrt {3}\, \left (-x \right )^{{3}/{2}}}{9} \\ y \left (x \right ) &= c_{1} \left (c_{1}^{2}+x \right ) \\ \end{align*}

1.78.5 Mathematica DSolve solution

Solving time : 0.006 (sec)
Leaf size : 54

DSolve[{y[x]==x*D[y[x],x]+(D[y[x],x])^3,{}}, 
       y[x],x,IncludeSingularSolutions->True]
 

\begin{align*} y(x)\to c_1 \left (x+c_1{}^2\right ) \\ y(x)\to -\frac {2 i x^{3/2}}{3 \sqrt {3}} \\ y(x)\to \frac {2 i x^{3/2}}{3 \sqrt {3}} \\ \end{align*}