1.5 problem 5

Internal problem ID [5]
Internal file name [OUTPUT/5_Sunday_June_05_2022_01_33_38_AM_47063498/index.tex]

Book: Differential equations and linear algebra, 3rd ed., Edwards and Penney
Section: Section 1.2. Integrals as general and particular solutions. Page 16
Problem number: 5.
ODE order: 1.
ODE degree: 1.

The type(s) of ODE detected by this program : "quadrature"

Maple gives the following as the ode type

[_quadrature]

\[ \boxed {y^{\prime }=\frac {1}{\sqrt {2+x}}} \] With initial conditions \begin {align*} [y \left (2\right ) = -1] \end {align*}

1.5.1 Existence and uniqueness analysis

This is a linear ODE. In canonical form it is written as \begin {align*} y^{\prime } + p(x)y &= q(x) \end {align*}

Where here \begin {align*} p(x) &=0\\ q(x) &=\frac {1}{\sqrt {2+x}} \end {align*}

Hence the ode is \begin {align*} y^{\prime } = \frac {1}{\sqrt {2+x}} \end {align*}

The domain of \(p(x)=0\) is \[ \{-\infty

1.5.2 Solving as quadrature ode

Integrating both sides gives \begin {align*} y &= \int { \frac {1}{\sqrt {2+x}}\,\mathop {\mathrm {d}x}}\\ &= 2 \sqrt {2+x}+c_{1} \end {align*}

Initial conditions are used to solve for \(c_{1}\). Substituting \(x=2\) and \(y=-1\) in the above solution gives an equation to solve for the constant of integration. \begin {align*} -1 = 4+c_{1} \end {align*}

The solutions are \begin {align*} c_{1} = -5 \end {align*}

Trying the constant \begin {align*} c_{1} = -5 \end {align*}

Substituting this in the general solution gives \begin {align*} y&=2 \sqrt {2+x}-5 \end {align*}

The constant \(c_{1} = -5\) gives valid solution.

Summary

The solution(s) found are the following \begin{align*} \tag{1} y &= 2 \sqrt {2+x}-5 \\ \end{align*}

(a) Solution plot

(b) Slope field plot

Verification of solutions

\[ y = 2 \sqrt {2+x}-5 \] Verified OK.

1.5.3 Maple step by step solution

\[ \begin {array}{lll} & {} & \textrm {Let's solve}\hspace {3pt} \\ {} & {} & \left [y^{\prime }=\frac {1}{\sqrt {2+x}}, y \left (2\right )=-1\right ] \\ \bullet & {} & \textrm {Highest derivative means the order of the ODE is}\hspace {3pt} 1 \\ {} & {} & y^{\prime } \\ \bullet & {} & \textrm {Integrate both sides with respect to}\hspace {3pt} x \\ {} & {} & \int y^{\prime }d x =\int \frac {1}{\sqrt {2+x}}d x +c_{1} \\ \bullet & {} & \textrm {Evaluate integral}\hspace {3pt} \\ {} & {} & y=2 \sqrt {2+x}+c_{1} \\ \bullet & {} & \textrm {Solve for}\hspace {3pt} y \\ {} & {} & y=2 \sqrt {2+x}+c_{1} \\ \bullet & {} & \textrm {Use initial condition}\hspace {3pt} y \left (2\right )=-1 \\ {} & {} & -1=2 \sqrt {4}+c_{1} \\ \bullet & {} & \textrm {Solve for}\hspace {3pt} c_{1} \\ {} & {} & c_{1} =-2 \sqrt {4}-1 \\ \bullet & {} & \textrm {Substitute}\hspace {3pt} c_{1} =-2 \sqrt {4}-1\hspace {3pt}\textrm {into general solution and simplify}\hspace {3pt} \\ {} & {} & y=2 \sqrt {2+x}-5 \\ \bullet & {} & \textrm {Solution to the IVP}\hspace {3pt} \\ {} & {} & y=2 \sqrt {2+x}-5 \end {array} \]

Maple trace

`Methods for first order ODEs: 
--- Trying classification methods --- 
trying a quadrature 
<- quadrature successful`
 

✓ Solution by Maple

Time used: 0.0 (sec). Leaf size: 13

dsolve([diff(y(x),x) = 1/(2+x)^(1/2),y(2) = -1],y(x), singsol=all)
 

\[ y \left (x \right ) = 2 \sqrt {2+x}-5 \]

✓ Solution by Mathematica

Time used: 0.008 (sec). Leaf size: 16

DSolve[{y'[x] == 1/(2+x)^(1/2),y[2]==-1},y[x],x,IncludeSingularSolutions -> True]
 

\[ y(x)\to 2 \sqrt {x+2}-5 \]