Primitive of x cubed over Root of x squared minus a squared
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Theorem
- $\ds \int \frac {x^3 \rd x} {\sqrt {x^2 - a^2} } = \frac {\paren {\sqrt {x^2 - a^2} }^3} 3 + a^2 \sqrt {x^2 - a^2} + C$
for $\size x > a$.
Proof
With a view to expressing the problem 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:
\(\ds u\) | \(=\) | \(\ds x^2\) | ||||||||||||
\(\ds \leadsto \ \ \) | \(\ds \frac {\d u} {\d x}\) | \(=\) | \(\ds 2 x\) | Power Rule for Derivatives |
and let:
\(\ds \frac {\d v} {\d x}\) | \(=\) | \(\ds \frac x {\sqrt {x^2 - a^2} }\) | ||||||||||||
\(\ds \leadsto \ \ \) | \(\ds v\) | \(=\) | \(\ds \sqrt {x^2 - a^2}\) | Primitive of $\dfrac x {\sqrt {x^2 - a^2} }$ |
Then:
\(\ds \int \frac {x^3 \rd x} {\sqrt {x^2 - a^2} }\) | \(=\) | \(\ds \int x^2 \frac {x \rd x} {\sqrt {x^2 - a^2} }\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds x^2 \sqrt {x^2 - a^2} - \int 2 x \sqrt {x^2 - a^2} \rd x\) | Integration by Parts | |||||||||||
\(\ds \) | \(=\) | \(\ds x^2 \sqrt {x^2 - a^2} - 2 \paren {\frac {\paren {\sqrt {x^2 - a^2} }^3} 3} + C\) | Primitive of $x \sqrt {x^2 - a^2}$ | |||||||||||
\(\ds \) | \(=\) | \(\ds \paren {x^2 - a^2 + a^2} \sqrt {x^2 - a^2} - 2 \paren {\frac {\paren {\sqrt {x^2 - a^2} }^3} 3} + C\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \paren {\sqrt {x^2 - a^2} }^3 + a^2 \sqrt {x^2 - a^2} - 2 \paren {\frac {\paren {\sqrt {x^2 - a^2} }^3} 3} + C\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \frac {\paren {\sqrt {x^2 - a^2} }^3} 3 + a^2 \sqrt {x^2 - a^2} + C\) |
$\blacksquare$
Also see
Sources
- 1968: Murray R. Spiegel: Mathematical Handbook of Formulas and Tables ... (previous) ... (next): $\S 14$: Integrals involving $\sqrt {x^2 - a^2}$: $14.212$