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

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19.4 First-order linear equations: use of an integrating factor 547

19.4 FIRST-ORDERLINEAREQUATIONS:USEOFAN

INTEGRATINGFACTOR

Inthissection wedevelop a method for solvingfirst-order linear differential equations.

19.4.1 Exactequations

Consider the differential equation

dy

dx = 3x2

This can be solved very easily by simplyintegrating both sidestogive

y = 3x 2 dx

=x 3 +c

wherecis the constant of integration. An equation which can be solved by integrating

both sides is said to be an exact equation. A more complicated example which, nevertheless,can

besolved inthe same way is

d

(xy) = 3x2

dx

Integrating both sides wefind

xy = 3x 2 dx

=x 3 +c

and dividing through byxgives the general solution

y=x 2 + c x

The differential equation we have justsolved isan exact equation.

Example19.12 Solve the equation

d (

x 2 y ) =cosx

dx

Solution Integrating both sides wefind

x 2 y = cosxdx

so that

=sinx+c

y = sinx

x 2 + c x 2

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