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2.1.10 Problem 10

Solved as second order missing y ode
✓Maple
✓Mathematica
✓Sympy

Internal problem ID [10369]
Book : Second order enumerated odes
Section : section 1
Problem number : 10
Date solved : Thursday, November 27, 2025 at 10:35:31 AM
CAS classification : [[_2nd_order, _quadrature]]

Solved as second order missing y ode

Time used: 0.512 (sec)

Solve

\begin{align*} {y^{\prime \prime }}^{2}&=x \\ \end{align*}
This is second order ode with missing dependent variable \(y\). Let
\begin{align*} u(x) &= y^{\prime } \end{align*}

Then

\begin{align*} u'(x) &= y^{\prime \prime } \end{align*}

Hence the ode becomes

\begin{align*} {u^{\prime }\left (x \right )}^{2}-x = 0 \end{align*}

Which is now solved for \(u(x)\) as first order ode.

Solve Multiplying the right side of the ode, which is \(\sqrt {x}\) by \(\frac {x}{u}\) gives

\begin{align*} u^{\prime }\left (x \right ) &= \left (\frac {x}{u}\right ) \sqrt {x}\\ &= \frac {x^{{3}/{2}}}{u}\\ &= F(x,u) \end{align*}

Since \(F \left (x , u\right )\) has \(u\), then let

\begin{align*} f_x&= x \left (\frac {\partial }{\partial x}F \left (x , u\right )\right )\\ &= \frac {3 x^{{3}/{2}}}{2 u}\\ f_u&= u \left (\frac {\partial }{\partial u}F \left (x , u\right )\right )\\ &= -\frac {x^{{3}/{2}}}{u}\\ \alpha &= \frac {f_x}{f_u} \\ &=-{\frac {3}{2}} \end{align*}

Since \(\alpha \) is independent of \(x,u\) then this is Homogeneous type G.

Let

\begin{align*} u&=\frac {z}{x^ \alpha }\\ &=\frac {z}{\frac {1}{x^{{3}/{2}}}} \end{align*}

Substituting the above back into \(F(x,u)\) gives

\begin{align*} F \left (z \right ) &=\frac {1}{z} \end{align*}

We see that \(F \left (z \right )\) does not depend on \(x\) nor on \(u\). If this was not the case, then this method will not work.

Therefore, the implicit solution is given by

\begin{align*} \ln \left (x \right )- c_1 - \int ^{u x^\alpha } \frac {1}{z \left (\alpha + F(z)\right ) } \,dz & = 0 \end{align*}

Which gives

\[ \ln \left (x \right )-c_1 +\int _{}^{\frac {u \left (x \right )}{x^{{3}/{2}}}}\frac {1}{z \left (\frac {3}{2}-\frac {1}{z}\right )}d z = 0 \]
Multiplying the right side of the ode, which is \(-\sqrt {x}\) by \(\frac {x}{u}\) gives
\begin{align*} u^{\prime }\left (x \right ) &= \left (\frac {x}{u}\right ) -\sqrt {x}\\ &= -\frac {x^{{3}/{2}}}{u}\\ &= F(x,u) \end{align*}

Since \(F \left (x , u\right )\) has \(u\), then let

\begin{align*} f_x&= x \left (\frac {\partial }{\partial x}F \left (x , u\right )\right )\\ &= -\frac {3 x^{{3}/{2}}}{2 u}\\ f_u&= u \left (\frac {\partial }{\partial u}F \left (x , u\right )\right )\\ &= \frac {x^{{3}/{2}}}{u}\\ \alpha &= \frac {f_x}{f_u} \\ &=-{\frac {3}{2}} \end{align*}

Since \(\alpha \) is independent of \(x,u\) then this is Homogeneous type G.

Let

\begin{align*} u&=\frac {z}{x^ \alpha }\\ &=\frac {z}{\frac {1}{x^{{3}/{2}}}} \end{align*}

Substituting the above back into \(F(x,u)\) gives

\begin{align*} F \left (z \right ) &=-\frac {1}{z} \end{align*}

We see that \(F \left (z \right )\) does not depend on \(x\) nor on \(u\). If this was not the case, then this method will not work.

Therefore, the implicit solution is given by

\begin{align*} \ln \left (x \right )- c_1 - \int ^{u x^\alpha } \frac {1}{z \left (\alpha + F(z)\right ) } \,dz & = 0 \end{align*}

Which gives

\[ \ln \left (x \right )-c_2 +\int _{}^{\frac {u \left (x \right )}{x^{{3}/{2}}}}\frac {1}{z \left (\frac {3}{2}+\frac {1}{z}\right )}d z = 0 \]
In summary, these are the solution found for \(y\)
\begin{align*} \ln \left (x \right )-c_1 +\int _{}^{\frac {u \left (x \right )}{x^{{3}/{2}}}}\frac {1}{z \left (\frac {3}{2}-\frac {1}{z}\right )}d z &= 0 \\ \ln \left (x \right )-c_2 +\int _{}^{\frac {u \left (x \right )}{x^{{3}/{2}}}}\frac {1}{z \left (\frac {3}{2}+\frac {1}{z}\right )}d z &= 0 \\ \end{align*}
For solution \(\ln \left (x \right )-c_1 +\int _{}^{\frac {u \left (x \right )}{x^{{3}/{2}}}}\frac {1}{z \left (\frac {3}{2}-\frac {1}{z}\right )}d z = 0\), since \(u=y^{\prime }\) then we now have a new first order ode to solve which is
\begin{align*} \ln \left (x \right )-c_1 +\int _{}^{\frac {y^{\prime }}{x^{{3}/{2}}}}\frac {1}{z \left (\frac {3}{2}-\frac {1}{z}\right )}d z = 0 \end{align*}

Solve Since the ode has the form \(y^{\prime }=f(x)\), then we only need to integrate \(f(x)\).

\begin{align*} \int {dy} &= \int {\operatorname {RootOf}\left (-\ln \left (x \right )+c_1 -\int _{}^{\frac {\textit {\_Z}}{x^{{3}/{2}}}}\frac {2}{3 z -2}d z \right )\, dx}\\ y &= \int \operatorname {RootOf}\left (-\ln \left (x \right )+c_1 -\int _{}^{\frac {\textit {\_Z}}{x^{{3}/{2}}}}\frac {2}{3 z -2}d z \right )d x + c_3 \end{align*}
\begin{align*} y&= \int \operatorname {RootOf}\left (-\ln \left (x \right )+c_1 -\int _{}^{\frac {\textit {\_Z}}{x^{{3}/{2}}}}\frac {2}{3 z -2}d z \right )d x +c_3 \end{align*}

In summary, these are the solution found for \((y)\)

\begin{align*} y &= \int \operatorname {RootOf}\left (-\ln \left (x \right )+c_1 -\int _{}^{\frac {\textit {\_Z}}{x^{{3}/{2}}}}\frac {2}{3 z -2}d z \right )d x +c_3 \\ \end{align*}
For solution \(\ln \left (x \right )-c_2 +\int _{}^{\frac {u \left (x \right )}{x^{{3}/{2}}}}\frac {1}{z \left (\frac {3}{2}+\frac {1}{z}\right )}d z = 0\), since \(u=y^{\prime }\) then we now have a new first order ode to solve which is
\begin{align*} \ln \left (x \right )-c_2 +\int _{}^{\frac {y^{\prime }}{x^{{3}/{2}}}}\frac {1}{z \left (\frac {3}{2}+\frac {1}{z}\right )}d z = 0 \end{align*}

Solve Since the ode has the form \(y^{\prime }=f(x)\), then we only need to integrate \(f(x)\).

\begin{align*} \int {dy} &= \int {\operatorname {RootOf}\left (-\ln \left (x \right )+c_2 -\int _{}^{\frac {\textit {\_Z}}{x^{{3}/{2}}}}\frac {2}{3 z +2}d z \right )\, dx}\\ y &= \int \operatorname {RootOf}\left (-\ln \left (x \right )+c_2 -\int _{}^{\frac {\textit {\_Z}}{x^{{3}/{2}}}}\frac {2}{3 z +2}d z \right )d x + c_4 \end{align*}
\begin{align*} y&= \int \operatorname {RootOf}\left (-\ln \left (x \right )+c_2 -\int _{}^{\frac {\textit {\_Z}}{x^{{3}/{2}}}}\frac {2}{3 z +2}d z \right )d x +c_4 \end{align*}

In summary, these are the solution found for \((y)\)

\begin{align*} y &= \int \operatorname {RootOf}\left (-\ln \left (x \right )+c_2 -\int _{}^{\frac {\textit {\_Z}}{x^{{3}/{2}}}}\frac {2}{3 z +2}d z \right )d x +c_4 \\ \end{align*}

Summary of solutions found

\begin{align*} y &= \int \operatorname {RootOf}\left (-\ln \left (x \right )+c_1 -\int _{}^{\frac {\textit {\_Z}}{x^{{3}/{2}}}}\frac {2}{3 z -2}d z \right )d x +c_3 \\ y &= \int \operatorname {RootOf}\left (-\ln \left (x \right )+c_2 -\int _{}^{\frac {\textit {\_Z}}{x^{{3}/{2}}}}\frac {2}{3 z +2}d z \right )d x +c_4 \\ \end{align*}
✓ Maple. Time used: 0.007 (sec). Leaf size: 27
ode:=diff(diff(y(x),x),x)^2 = x; 
dsolve(ode,y(x), singsol=all);
\begin{align*} y &= \frac {4 x^{{5}/{2}}}{15}+c_1 x +c_2 \\ y &= -\frac {4 x^{{5}/{2}}}{15}+c_1 x +c_2 \\ \end{align*}

Maple trace 

Methods for second order ODEs: 
Successful isolation of d^2y/dx^2: 2 solutions were found. Trying to solve each\ 
 resulting ODE. 
   *** Sublevel 2 *** 
   Methods for second order ODEs: 
   --- Trying classification methods --- 
   trying a quadrature 
   <- quadrature successful 
------------------- 
* Tackling next ODE. 
   *** Sublevel 2 *** 
   Methods for second order ODEs: 
   --- Trying classification methods --- 
   trying a quadrature 
   <- quadrature successful
✓ Mathematica. Time used: 0.003 (sec). Leaf size: 41
ode=(D[y[x],{x,2}])^2==x; 
ic={}; 
DSolve[{ode,ic},y[x],x,IncludeSingularSolutions->True]
\begin{align*} y(x)&\to -\frac {4 x^{5/2}}{15}+c_2 x+c_1\\ y(x)&\to \frac {4 x^{5/2}}{15}+c_2 x+c_1 \end{align*}
✓ Sympy. Time used: 0.231 (sec). Leaf size: 31
from sympy import * 
x = symbols("x") 
y = Function("y") 
ode = Eq(-x + Derivative(y(x), (x, 2))**2,0) 
ics = {} 
dsolve(ode,func=y(x),ics=ics)
\[ \left [ y{\left (x \right )} = C_{1} + C_{2} x - \frac {4 x^{\frac {5}{2}}}{15}, \ y{\left (x \right )} = C_{1} + C_{2} x + \frac {4 x^{\frac {5}{2}}}{15}\right ] \]

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