Tuesday, September 24, 2013

ylnx - xy' = 0 Find the general solution of the differential equation

For the given problem: yln(x)-xy'=0 , we can evaluate this by applying variable separable differential equation in which we express it in a form of f(y) dy = f(x)dx .
to able to apply direct integration:  int f(y) dy = int f(x)dx .
Rearranging the problem:
yln(x)-xy'=0
yln(x)=xy'  or xy' = y ln(x)
(xy')/(yx) = (y ln(x))/(yx)
(y') /y = ln(x)/x
Applying direct integration, we denote y' = (dy)/(dx) :
int (y') /y = int ln(x)/x
int 1 /y (dy)/(dx) = int ln(x)/x
int 1 /y (dy)= int ln(x)/x dx
 
For the left side, we apply the basic integration formula for logarithm: int (du)/u = ln|u|+C
int 1 /y (dy) = ln|y|
For the right side, we apply u-substitution by letting u= ln(x) then du = 1/x dx .
int ln(x)/x dx=int udu
 Applying the Power Rule for integration : int x^n= x^(n+1)/(n+1)+C .
int udu=u^(1+1)/(1+1)+C
          =u^2/2+C
Plug-in u = ln(x) in u^2/2+C , we get:
int ln(x)/x dx =(ln(x))^2/2+C
Combining the results, we get the general solution for differential equation (yln(x)-xy'=0)  as:
ln|y|=(ln|x|)^2/2+C
 
The general solution: ln|y|=(ln|x|)^2/2+C can be expressed as:
y = C_1e^((ln|x|)^2/2)+C .

No comments:

Post a Comment

Why is the fact that the Americans are helping the Russians important?

In the late author Tom Clancy’s first novel, The Hunt for Red October, the assistance rendered to the Russians by the United States is impor...