Reduction Formula for Power of x by General Exponential of a x
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Theorem
- $\ds \int x^n b^{a x} \rd x = \frac {x^n b^{a x} } {a \ln b} - \dfrac n {a \ln b} \int x^{n - 1} b^{a x} \rd x$
for $n \in \Z_{>0}$, $a \in \R_{\ne 0}$, $b > 0$, $b \ne 1$
Proof
With a view to expressing the primitive in the form:
- $\ds \int u \frac {\d v} {\d x} \rd x = u v - \int v \frac {\d u} {\d x} \rd x$
let:
\(\ds u\) | \(=\) | \(\ds x^n\) | ||||||||||||
\(\ds \leadsto \ \ \) | \(\ds \frac {\d u} {\d x}\) | \(=\) | \(\ds n x^{n - 1}\) | Power Rule for Derivatives |
and let:
\(\ds \frac {\d v} {\d x}\) | \(=\) | \(\ds b^{a x}\) | ||||||||||||
\(\ds \leadsto \ \ \) | \(\ds v\) | \(=\) | \(\ds \dfrac {e^{a x} } {a \ln b}\) | Primitive of $b^{a x}$ |
Then:
\(\ds \int x^n b^{a x} \rd x\) | \(=\) | \(\ds \int u \rd v\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \paren {x^n} \paren {\dfrac {b^{a x} } {a \ln b} } - \int \paren {\dfrac {b^{a x} } {a \ln b} } \paren {n x^{n - 1} } \rd x\) | Integration by Parts | |||||||||||
\(\ds \) | \(=\) | \(\ds \frac {x^n b^{a x} } {a \ln b} - \dfrac n {a \ln b} \int x^{n - 1} b^{a x} \rd x\) | simplifying |
$\blacksquare$
Sources
- 1968: George B. Thomas, Jr.: Calculus and Analytic Geometry (4th ed.) ... (previous) ... (next): Back endpapers: A Brief Table of Integrals: $106$.