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...Variation of parameters the method applies to solve a x y b c f continuity and is assumed plus important because it solves largest class equations specically included are functions like ln e homogeneous equation uses facts about dierential success depends upon writing general solution as where known arbitrary constants if then standard recipe for nds that reported by independent independence twosolutionsy yof arecalledindependentif neither constant multiple other term dependent means not in which case either cy or holds all some can be tested through wronskian dened w theorem thewronskianoftwosolutionssatisesa whichimplies abel s identity rx t dt two solutions only proof appears on page formula let continuous near ay non has particular yp z dx delayed history was dis covered varying assuming they depend this results formulas r see historical details substitute use collect expand yy any one last three equivalent called classical vari ation fraction integrand cauchy kernel we prefer rst ...