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I just learned today how to solve a first-order nonhomogeneous equation by breaking it down into a particular integral and a complementary function. What i'm wondering is WHY is the solution equal to these 2 things added together? I can see how each individual part of them is solved from the general term but I dont understand why when u add them together, u get the general solution to the complete differential equation?

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    0% accepted rate is not a good sign here. :)2012-10-23

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Are you asking why the general solution is composed of a homogeneous solution and a particular solution? All right, let's say we have a particular solution $y_p$ that satisfies the linear differential equation

$\sum f_n(x)\frac{d^ny_p}{dx^n}=g(x)$

and a corresponding homogeneous solution $y_h$ satisfying

$\sum f_n(x)\frac{d^ny_h}{dx^n}=0$

Now because the derivative of a sum is equal to the sum of the derivatives, we have

$\sum f_n(x)\frac{d^n(y_p+y_h)}{dx^n}=\sum f_n(x)\frac{d^ny_p}{dx^n}+\sum f_n(x)\frac{d^ny_h}{dx^n}=g(x)+0=g(x)$