Wednesday 26 August 2015

`xdy = (x + y + 2)dx , y(1) = 10` Find the particular solution of the differential equation that satisfies the initial condition

Given ` xdy = (x + y + 2)dx `


=> `xdy/dx = (x + y + 2)`


=> `y'=1+y/x+2/x`


=> `y'-y/x = 1+2/x`


=> `y'-y/x =(x+2)/x`


when the first order linear ordinary differential equation has the form of


`y'+p(x)y=q(x)`


then the general solution is ,


`y(x)=((int e^(int p(x) dx) *q(x)) dx +c)/e^(int p(x) dx)`


so,


`y'-y/x =(x+2)/x--------(1)`


`y'+p(x)y=q(x)---------(2)`


on comparing both we get,


`p(x) = -1/x and q(x)=(x+2)/x`


so on solving with the above...

Given ` xdy = (x + y + 2)dx `


=> `xdy/dx = (x + y + 2)`


=> `y'=1+y/x+2/x`


=> `y'-y/x = 1+2/x`


=> `y'-y/x =(x+2)/x`


when the first order linear ordinary differential equation has the form of


`y'+p(x)y=q(x)`


then the general solution is ,


`y(x)=((int e^(int p(x) dx) *q(x)) dx +c)/e^(int p(x) dx)`


so,


`y'-y/x =(x+2)/x--------(1)`


`y'+p(x)y=q(x)---------(2)`


on comparing both we get,


`p(x) = -1/x and q(x)=(x+2)/x`


so on solving with the above general solution we get:


y(x)=`((int e^(int p(x) dx) *q(x)) dx +c)/e^(int p(x) dx)`


=`((int e^(int (-1/x) dx) *((x+2)/x)) dx +c)/e^(int (-1/x) dx)`


first we shall solve


`e^(int (-1/x) dx)=e^(ln(1/x)) = (1/x)`     


so


proceeding further, we get


y(x) =`((int e^(int (-1/x) dx) *((x+2)/x)) dx +c)/e^(int (-1/x) dx)`


=`((int (1/x) *((x+2)/x)) dx +c)/(1/x)`


=`(int (1/x) *((1+(2)/x)) dx +c)/(1/x)`


=` x(ln(x)-2/x +c)`


so now let us find the particular solution of differential equation at y(1)=10


`y(1) = 1(ln(1)-2/1 +c)`


=> `10 = 0-2+c`


`c=12`


`y(x) =x(ln(x)-2/x +12)` is the solution

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