Sunday 20 April 2014

`dy/dx = (1-2x) / (4x-x^2)` Solve the differential equation

`dy/dx = (1-2x) / (4x-x^2)`


This differential equation is separable since it can be written in the form



  • `N(y)dy =M(x)dx`

Bringing together same variables on one side, the equation becomes


`dy=(1-2x)/(4x-x^2)dx`


Taking the integral of both sides, it turns into


`int dy = int(1-2x)/(4x-x^2)dx`


`y+C_1 = int(1-2x)/(4x-x^2)dx`


`y+C_1= int (1-2x)/(x(4-x))dx`


To take the integral of right side, apply partial fraction decomposition.


  • `(1-2x)/(x(4-x)) = A/x + B/(4-x)`

  • `1-2x = A(4-x)+Bx`

Let x=0.



`1-2(0) = A(4-0)+B(0)`


`1=4A`


...


`dy/dx = (1-2x) / (4x-x^2)`


This differential equation is separable since it can be written in the form



  • `N(y)dy =M(x)dx`

Bringing together same variables on one side, the equation becomes


`dy=(1-2x)/(4x-x^2)dx`


Taking the integral of both sides, it turns into


`int dy = int(1-2x)/(4x-x^2)dx`


`y+C_1 = int(1-2x)/(4x-x^2)dx`


`y+C_1= int (1-2x)/(x(4-x))dx`


To take the integral of right side, apply partial fraction decomposition.


  • `(1-2x)/(x(4-x)) = A/x + B/(4-x)`


  • `1-2x = A(4-x)+Bx`


Let x=0.




`1-2(0) = A(4-0)+B(0)`


`1=4A`


`1/4=A`


Let x=4.


`1-2(4)=A(4-4)+B(4)`


`-7=4B`


`-7/4=B`



  • `1/(2x-x^2) = (1/4)/x + (-7/4)/(4-x)`


  • `1/(2x-x^2) = 1/(4x) + (-7)/(4(4-x))`


  • `1/(2x-x^2) = 1/(4x) + (-7)/(-4(x-4))`


  • `1/(2x-x^2) = 1/(4x) + (7)/(4(x-4))`

So the integrand at the right side decomposes to


`y + C_1 = int (1/(4x) + 7/(4(x-4)))dx`


Then, apply the formula `int 1/u du = ln|u| + C` .


`y + C_1 = 1/4ln|x| + 7/4ln|x-4|+C_2`


Isolating the y, the equation becomes


`y= 1/4ln|x| + 7/4ln|x-4|+C_2-C_1`


Since C1 and C2 represent any number, it can be expressed as a single constant C.


`y = 1/4ln|x| + 7/4ln|x-4|+C`



Therefore, the general solution of the given differential equation is `y = 1/4ln|x| + 7/4ln|x-4|+C` .

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