Monday, September 26, 2016

`(2+x)y' = 3y` Find the general solution of the differential equation

Recall that `y'` is the same as `(dy)/(dx)` . Then in the given problem: `(2+x)y'=3y` , we may write it as:


`(2+x) (dy)/(dx) = 3y.`


 This will help to follow the variable separable differential equation in a form of `N(y) dy = M(x) dx.`


To rearrange `(2+x) (dy)/(dx) = 3y` ,cross-multiply `(dx)` to the other side:


`(2+x)dy =3y dx`


Divide both sides by `(2+x)` :


`((2+x)dy)/(2+x) =(3y dx)/(2+x)`


`dy =(3y dx)/(2+x)`


Divide both sides by `y` :


`(dy )/y=(3y dx)/((2+x)y)`


`(dy)/y=(3dx)/(2+x)`


 To solve for the general solution of the differential equation, apply direct integration on both sides:


`int (dy)/y=int (3dx)/(2+x)`


For the left side, apply the basic integration formula for logarithm


`int (dy)/y= ln|y|`


For the right side, we may apply the basic integration property: `int c*f(x) dx = c int f(x)dx` .


`int (3dx)/(2+x)= 3 int (dx)/(2+x)`


 Let `u =2+x` then du= dx


The integral becomes:


`3 int (dx)/(2+x) = 3 int (du)/u`


We can now apply the  basic integration formula for logarithm on the integral part:


`3 int (du)/u= 3ln|u| +C`


Recall `u =(2+x) ` then `3 int (dx)/(2+x) =3ln|2+x| +C`


Combining the results from both sides, we get:


`ln|y|=3ln|2+x| +C`


`y=e^(3ln|x+2|+C)`


` y= e^(ln(x+2)^3+C)` 


Law of Exponents:` x^(n+m)= x^n*x^m`


`y= e^(ln(x+2)^3)*e^C`


`e^C =C` is an arbitrary constant, so


`y= Ce^(ln(x+2)^3)`


` y = C(x+2)^3`

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