tx ty t f x y n m xydx nxydy - uhalmus/4389_de_summary_misc...homogeneous equations a function of...
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Homogeneous Equations
A function of two variables f(x,y) is said to be homogeneous of degree n if there is a constant n such that
, ,nf tx ty t f x y for all t, x and y.
A differential equation of the form
( , ) ( , ) 0M x y dx N x y dy
Is homogeneous if M and N are both homogeneous functions of the same degree.
To solve homogeneous equations, turn them into separable equations using the substitution: y xv .
Example:
Solve the equation: 2 2 2 0x y dx xydy
Exact Equations
Given a function ,f x y , its total differential is: f f
df dx dyx y
.
Hence, the family of curves ,f x y c satisfy the differential equation
0f f
dx dyx y
So, if there is a function such that ( , )f
M x yx
and ( , )f
N x yy
, then
( , ) ( , ) 0M x y dx N x y dy is called and “exact differential equation”.
How can you decide that a differential equation is exact?
Check: ??M N
y x
Example:
Solve: 3 21 2 4 0xy dx y x dx
Non-Exact Equations and Integrating Factors
Example: 21 0xy x dx x dy
Let ( , ) ( , ) 0M x y dx N x y dy be a non-exact equation.
Case 1: If y xM N
N
is a function of “x” only, call this function ( )f x . The
integrating factor is f x dxx e .
Case 2: If y xM N
M
is a function of “y” only, call this function ( )f y . The
integrating factor is f y dyy e .
Example: Find the integral curve of
2
30
dy x y
dx x y
that passes through the point (1,1).
Linear Equations with Constant Coefficients:
Example:
Find the general solution:
a) '' 4 0y y
b) 2 '' 7 ' 4y y y
c) '' 2 ' 0y y y
d) ''' '' 9 ' 9 0y y y y
Example: Solve: ''y x y
Example: Find the Laplace Transform of 2xf x e .
Formula: 0
( ) ( )stL f e f t dt
.
Euler’s Equations (Can find this on Chapter 3 notes – 3.2)
Rewrite: 2
2 ( 1) 0 where lnd y dy
y z xdzdz
Example:
Solve 2 '' 4 ' 6 0x y xy y