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082-Engineering-Mathematics-Anthony-Croft-Robert-Davison-Martin-Hargreaves-James-Flint-Edisi-5-2017

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552 Chapter 19 Ordinary differential equations I

Insummary:

Given any first-order linear equation instandard form

dy

dx +Py=Q

wherePandQarefunctions ofx, the integrating factor µ isgiven by

µ = e ∫ Pdx

and the solution of the equation isthen obtained from

µy = µQdx

It is important when working on a particular differential equation to rewrite the standard

formulae in the correct form before use. Essentially this means using the correct

dependentandindependentvariablesintheequations.Forexample,ifxisthedependent

variable andt isthe independent variable then the equations areas follows:

Theintegrating factorfor

dx

dt +Px=Q

wherePandQarefunctions oft, isgiven by

µ = e ∫ Pdt

and the solution of the equation isobtained from

µx = µQdt

These results areillustratedinthe examples which follow.

Example19.14 Solve the differential equation dy

dx + y = 1 using the integrating factor method.

x

Solution Referring tothe standard first-order linear equation

dy

+P(x)y =Q(x)

dx

weseethatP(x) = 1 andQ(x) = 1.Usingthe previous formula for µ(x), we find

x

µ(x) = e ∫ (1/x)dx

= e lnx

=x

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