1.

If \[\int\limits_{0}^{\infty }{{{e}^{-ax}}dx=\frac{1}{a},}\] then \[\int\limits_{0}^{\infty }{{{x}^{n}}{{e}^{-ax}}dx}\] is

A. \[\frac{{{(-1)}^{n}}n!}{{{a}^{n+1}}}\]
B. \[\frac{{{(-1)}^{n}}(n-1)!}{{{a}^{n}}}\]
C. \[\frac{n!}{{{a}^{n+1}}}\]     
D. None of these
Answer» D. None of these


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