Wednesday, March 26, 2014

`(du)/(dv) = uvsin(v^2) , u(0) = 1` Find the particular solution that satisfies the initial condition

An ordinary differential equation (ODE) has differential equation for a function with single variable. A first order ODE follows  .


In the given problem: `(du)/(dv)=uvsin(v^2)` ,  we may apply variable separable differential equation in a form of   .


Divide both sides by "u" and cross-multiply dv  to set it up as:


`(du)/u=vsin(v^2) dv.`


Apply direct integration: `int(du)/u=int vsin(v^2) dv.`


For the left sign, we follow the basic integration formula for logarithm:


`int (du)/u = ln|u|`


For the right side, we follow the basic integration formula for sine function:


Let: `w=v^2` then `dw = 2v*dv` or `(dw)/2 =v dv` .


The integral becomes:


`intvsin(v^2) dv= intsin(v^2) * vdv`


                       `=intsin(w) *(dw)/2`


                      `= (1/2) int sin(w) dw`


                      `= (1/2)*(-cos(w))+C`


                     ` =-cos(w)/2+C`


Plug-in `w=v^2` on `-cos(w)/2+C` , we get:


`intvsin(v^2) dv=-cos(v^2)/2+C`


Combing the results, we get the general solution of differential equation as:


`ln|u| = -cos(v^2)/2+C`



To solve for the arbitrary constant `(C)` , apply the initial condition `u(0)=1`  on`ln|u| = -cos(v^2)/2+C` :


`ln|1| = -cos(0^2)/2+C`


`0 = -1/2+C`


`C = 0+1/2`


`C=1/2`


Plug-in ` C= 1/2` in `ln|u| = -cos(v^2)/2+C` , we get 


`ln|u| = -cos(v^2)/2+1/2`


 `u = e^(-cos(v^2)/2+1/2)`

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