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

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thatis,

β = 3

32

The required particular integral isy P

(x) = x 8 + 3

32 .

19.6 Constant coefficient equations 571

Experienceleadstothe trialsolutionssuggestedinTable 19.1.

Table19.1

Trial solutionsto find the particular

integral.

f (x)

constant

polynomial inx

ofdegreer

coskx

sinkx

ae kx

Trialsolution

constant

polynomial inx

ofdegreer

acoskx +bsinkx

acoskx +bsinkx

αe kx

Example19.28 Findaparticular integralforthe equation

d 2 y

dx 2 −6dy dx +8y=3cosx

Solution We shall try a solution ofthe form

y P

(x)=αcosx+βsinx

Differentiating,we find

dy P

dx =−αsinx+βcosx

d 2 y P

dx 2 =−αcosx−βsinx

Substitution into the differential equation gives

(−αcosx−βsinx)−6(−αsinx+βcosx)+8(αcosx+βsinx)

=3cosx

Equating coefficients of cosxwefind

−α−6β+8α=3 (19.17)

whilethose of sinx give

−β+6α+8β=0 (19.18)

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