Monday, 14 September 2015

`y' + 2xy = 10x` Solve the first-order differential equation

`y'+2xy =10x`


To solve, re-write the derivative as `dy/dx` .


`dy/dx + 2xy = 10x`


Then, bring together same variables on one side of the equation.


`dy/dx = 10x - 2xy`


`dy/dx = 2x(5 - y)`


`dy/(5-y) = 2x dx`


Next, take the integral of both sides.


`int dy/(5-y) = int 2xdx`


`-ln |5-y| +C_1= (2x^2)/2 + C_2`


Then, isolate the y.


`-ln|5-y| = x^2+C_2-C_1`


`ln|5-y|=-x^2- C_2 +C_1`


Since C1 and C2 represent any number,...

`y'+2xy =10x`


To solve, re-write the derivative as `dy/dx` .


`dy/dx + 2xy = 10x`


Then, bring together same variables on one side of the equation.


`dy/dx = 10x - 2xy`


`dy/dx = 2x(5 - y)`


`dy/(5-y) = 2x dx`


Next, take the integral of both sides.


`int dy/(5-y) = int 2xdx`


`-ln |5-y| +C_1= (2x^2)/2 + C_2`


Then, isolate the y.


`-ln|5-y| = x^2+C_2-C_1`


`ln|5-y|=-x^2- C_2 +C_1`


Since C1 and C2 represent any number, express it as a single constant C.


`ln|5-y| = -x^2+ C`


`e^(ln|5-y|) = e^(-x^2+C)`


`|5-y| = e^(-x^2+C)`


`5-y = +-e^(-x^2+C)`


Applying the exponent rule `a^m*a^n = a^(m+n)` ,


the right side becomes


`5-y = +- e^(-x^2)*e^C`


`5-y = +-e^C*e^(-x^2)`


`-y = +-e^C*e^(-x^2) - 5`


`y = +-e^C*e^(-x^2)+5`


Since+-e^C is a constant, it can be replaced by a constant C.


`y = Ce^(-x^2) + 5`



Therefore, the general solution is  `y = Ce^(-x^2) + 5` .

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