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Integration using parametric derivatives : ウィキペディア英語版
Integration using parametric derivatives

In mathematics, integration by parametric derivatives is a method of integrating certain functions.
For example, suppose we want to find the integral
: \int_0^\infty x^2 e^ \, dx.
Since this is a product of two functions that are simple to integrate separately, repeated integration by parts is certainly one way to evaluate it. However, we may also evaluate this by starting with a simpler integral and an added parameter, which in this case is ''t'' = 3:
:
\begin
& \int_0^\infty e^ \, dx = \left(\frac \right )_0^\infty = \left( \lim_ \frac \right) - \left( \frac \right) \\
& = 0 - \left( \frac \right) = \frac.
\end

This converges only for ''t'' > 0, which is true of the desired integral. Now that we know
: \int_0^\infty e^ \, dx = \frac,
we can differentiate both sides twice with respect to ''t'' (not ''x'') in order to add the factor of ''x''2 in the original integral.
:
\begin
& \frac \int_0^\infty e^ \, dx = \frac \frac \\()
& \int_0^\infty \frac e^ \, dx = \frac \frac \\()
& \int_0^\infty \frac \left (-x e^\right) \, dx = \frac \left(-\frac\right) \\()
& \int_0^\infty x^2 e^ \, dx = \frac.
\end

This is the same form as the desired integral, where ''t'' = 3. Substituting that into the above equation gives the value:
: \int_0^\infty x^2 e^ \, dx = \frac = \frac.


抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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