Thursday, October 10, 2013

`y' = 5x/y` Solve the differential equation

The general solution of a differential equation in a form of `y'=f(x,y)` can be evaluated using direct integration. The derivative of y denoted as `y'` can be written as `(dy)/(dx)` then `y'= f(x)` can be expressed as `(dy)/(dx)= f(x)` .


For the problem: `y'=5x/y` , we let ` y'=(dy)/(dx) ` to set it up as:


`(dy)/(dx)= 5x/y`


Cross-multiply `dx` to the right side:


`(dy)= 5x/ydx`


Cross-multiply y to the left side:


`ydy=5xdx`


Apply direct integration on both sides:


`int ydy=int 5xdx`


Apply basic integration property:` int c*f(x)dx = c int f(x) dx` on the right side.


`int ydy=int 5xdx`


`int ydy=5int xdx`


Apply Power Rule for integration: `int u^n du= u^(n+1)/(n+1)+C` on both sides.


For the left side, we get:


`int y dy = y^(1+1)/(1+1)`


            `= y^2/2`


For the right side, we get:


`int x dx = x^(1+1)/(1+1)+C`


            `= x^2/2+C`


Note: Just include the constant of integration "C" on one side as the arbitrary constant of a differential equation.


Combining the results from both sides, we get the general solution of the differential equation as:


`y^2/2=x^2/2+C`


or` y =+-sqrt(x^2/2+C)`

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