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单词 ExampleOfChangingVariable
释义

example of changing variable


If one performs in the improper integral

I:=-ekx1+exdx  (0<k<1)(1)

the change of variable (http://planetmath.org/ChangeOfVariableInDefiniteIntegral)

x=-lnt,dx=-dtt,

the new lower limitMathworldPlanetmath becomes and the new upper limit 0; hence one obtains

I=-0e-klntdt(1+e-lnt)t=0t-kt+1𝑑t.

Thus one has recurred I to the integral

0x-kx+1𝑑x,(2)

the value of which has been determined in the entry using residue theoremMathworldPlanetmath near branch pointMathworldPlanetmath.  Accordingly, we may write the result

-ekx1+ex𝑑x=πsinπk.

Calculating the integral (1) directly is quite laborious:  one has to use Cauchy residue theorem to the integral

cekz1+ez𝑑z

about the perimetre c of the rectangle

-aReza,0Imz 2π

and then to let  a (one cannot use the same half-disk as in determining the integral (2)).  As for using the method (http://planetmath.org/MethodsOfEvaluatingImproperIntegrals) of differentiationMathworldPlanetmath under the integral sign or taking Laplace transformMathworldPlanetmath with respect to k yields a more complicated integral.

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更新时间:2025/5/4 13:07:56