FIRST-ORDER ORDINARY DIFFERENTIAL EQUATIONS G(x, y, y ′ )=0 ♦ in normal form : y ′ = F (x, y ) ♦ in differential form : M (x, y )dx + N (x, y )dy =0 • Last time we discussed first-order linear ODE: y ′ + q (x)y = h(x). We next consider first-order nonlinear equations.
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FIRST-ORDER ORDINARY DIFFERENTIAL EQUATIONS
G(x, y, y′) = 0
♦ in normal form:
y′ = F (x, y)
♦ in differential form:
M(x, y)dx+N(x, y)dy = 0
• Last time we discussed first-order linear ODE: y′ + q(x)y = h(x).We next consider first-order nonlinear equations.
NONLINEAR FIRST-ORDER ODEs
• No general method of solution for 1st-order ODEs beyond linear case;rather, a variety of techniques that work on a case-by-case basis.
Examples:
i) Bring equation to separated-variables form, that is, y′ = α(x)/β(y);
then equation can be integrated.
Cases covered by this include y′ = ϕ(ax+ by); y′ = ϕ(y/x).
ii) Reduce to linear equation by transformation of variables.
Examples of this include Bernoulli’s equation.
iii) Bring equation to exact-differential form, that is
M(x, y)dx+N(x, y)dy = 0 such that M = ∂φ/∂x, N = ∂φ/∂y.
Then solution determined from φ(x, y) = const.
• Useful reference for the ODE part of this course(worked problems and examples)
Schaum’s Outline Series
Differential Equations
R. Bronson and G. CostaMcGraw-Hill (Third Edition, 2006)
♦ Chapters 1 to 7: First-order ODE.
First order nonlinear equations
Although no general method for solution is available, there are several cases of
physically relevant nonlinear equations which can be solved analytically :
Separable equations
d ( )
d ( )
y f x
x g y=
( ) ( )g y dy f x dx=! !Solution :
2de
d
xyy
x=Ex 1
2e dxdyx
y=! " "
1exc
y
!= +i.e
1
(e )xy
c
!=
+or
Almost separable equations
d( )
d
yf ax by
x= +
z ax by= +dd
d d
yz
x xa b= +Change variables :
1d
( ( ))x z
a bf z= .
+!d
( )d
za bf z
x= + !
2d( 4 )
d
yx y
x= ! + 4z y x= !
2d d4 4
d d
z yz
x x! = " + = "
1 24 2ln( ) Cz
zx
!
+= +
4
4
(1 e )
(1 e )4 2
x
x
k
ky x
+
!" = +
Ex 2
k a constant
z ax by= +
d( )
d
za bf z
x= + !
d( )
d
yf y x
x= / .
Homogeneous equations
The equation is invariant under , .. h.. omogeneousx sx y sy! !
v v' vy x y x!= " = + .Solution
d(v) v
d ln constantv
f
xx
x! = = + ." "
1. . v ' ( (v) v)i e f
x= !
2 2d( ) e
d
y xyxy y x y
x
! /! = +Ex 3
2 vv
2
(1 v) e vdv(v v) v e ln
v (1 v)x x
!+" + ! = # = .
+$
1 vu ! +
1 1
2
1 1 ee ( )e d e [ ]
u
uu
u u u
! !! = ."
v v vy x y x! != " = + .Change variables
To evaluate integral change variables
. . ln1
y
x
y
x
ei e x =
+
Homogeneous
Homogeneous but for constants
2 1
2
dy x y
dx x y
+ +=
+ +
' , 'x x a y y b= + = +' ' ' '
.' '
dy dy dy dx dy
dx dx dx dx dx! = = =
' ' 2 ' 1 2
' ' ' 2
dy x y a b
dx x y a b
+ + + +=
+ + + +
1 2 0a b+ + =
2 0a b+ + =
3, 1a b= ! =
' ' 2 '
' ' '
dy x y
dx x y
+=
+Homogeneous
The Bernoulli equation
d( ) ( ) , 1
d
nyP x y Q x y n
x+ = !
To solve, change variable to 1 n
z y!
= (1 ) ndz dyn y
dx dx
!" = !
(1 ) ( ) (1 ) ( )dz
n P x z n Q xdx
+ ! = !Gives the equation
Ex 42/3
'y y y+ =1 1/3n
z y y!
= = 1
3'3
zz! + =
x/3Integrating factor e
1/ 3 /31
xz y ce
!= = +
/ 3 /3/ 3
x xze e dx! = "
1st order Linear
1st order linear
Exercise:
Solve the equation 2 y ′ = y/x+ x2/ywith initial condition y(1) = 2.
• This equation is Bernoulli with n = −1.
• Set z = y2. Then z′ − z/x = x2.
• Integrating factor I(x) = 1/x
⇒ z(x) = x [
∫
dx x2/x+ const.] = x3/2 + const. x
Thus y = z1/2 = ±√
x3/2 + const. x
• Initial condition y(1) = 2
⇒ y(x) =
√
x3 + 7x
2
Exact equations
• A first-order ODE
M(x, y)dx+N(x, y)dy = 0
is exact if there exists a function φ(x, y) such that
∂φ
∂x= M ,
∂φ
∂y= N .
• In this case the differential equation can be recast as
dφ = M(x, y)dx+N(x, y)dy = 0
so that the solution to it is determined by
φ(x, y) = constant .
Example: Solve the equation xy′ = −2 tan y.
• This equation can be rewritten as
2x sin y dx+ x2 cos y dy = 0 ,
i.e., M(x, y) = 2x sin y , N(x, y) = x2 cos y ,
which is exact because∂φ
∂x= 2x sin y ⇒ φ(x, y) = x2 sin y + α(y)
∂φ
∂y= x2 cos y ⇒ x2 cos y + α′(y) = x2 cos y ⇒ α = constant
• Therefore φ(x, y) = x2 sin y + c,and the general solution is determined by x2 sin y = const.:
⇒ y(x) = arcsin(
const./x2)
DIFFERENTIAL EQUATIONS AND FAMILIES OF CURVES
• General solution of a first-order ODE y′ = f(x, y)
contains an arbitrary constant: y = (x, c)
⊲ one curve in x, y plane for each value of c⊲ general solution can be thought of as one-parameter family of curves
Example: y′ = −x/y.
separable equation ⇒
∫
y dy = −
∫
x dx ⇒ y2/2 = −x2/2 + c
i.e., x2 + y2 = constant : family of circles centered at origin
Fig.1
x
y
Orthogonal trajectories
• Given the family of curves representing solutions of ODE y′ = f(x, y),
orthogonal trajectories are given by a second family of curves which are
solutions of
y′ = −1/f(x, y).
♦ Then each curve in either family is perpendicularto every curve in the other family.
Example:
Find the orthogonal trajectories to the family of circles y′ = −x/y.
• Solve y′ = y/x .
⇒
∫
dy
y=
∫
dx
x⇒ ln y = ln x+ constant
i.e., y = cx : family of straight lines through the origin
a) Find the family of curves corresponding to solutions of the ODE
y′ = (y2 − x2)/(2xy).
b) Find the orthogonal trajectories to the above family of curves.