(Variation of parameter) Another method of obtaining (4) results from the following
Chapter 1, Problem 1.1.147(choose chapter or problem)
Another method of obtaining (4) results from the following idea. Write (3) as cy*, where y* is the exponential function. which is a solution of the homogeneous linear ODE y*’ + py* = 0. Replace the arbitrary constant c in (3) with a function u to be determined so that the resulting function y = uy* is a solution of the nonhomogeneous linear ODE y ‘ + py = r.
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