Primitive of Fourth Power of Cosine of a x
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Theorem
- $\ds \int \cos^4 a x \rd x = \frac {3 x} 8 + \frac {\sin 2 a x} {4 a} + \frac {\sin 4 a x} {32 a} + C$
Proof
\(\ds \int \cos^4 a x \rd x\) | \(=\) | \(\ds \int \paren {\frac {3 + 4 \cos 2 x + \cos 4 x} 8} \rd x\) | Power Reduction Formula for $\cos^4$ | |||||||||||
\(\ds \) | \(=\) | \(\ds \frac 3 8 \int \d x + \frac 1 2 \int \cos 2 a x \rd x + \frac 1 8 \int \cos 4 a x \rd x\) | Linear Combination of Primitives | |||||||||||
\(\ds \) | \(=\) | \(\ds \frac {3 x} 8 + \frac 1 2 \int \cos 2 a x \rd x + \frac 1 8 \int \cos 4 a x \rd x + C\) | Primitive of Constant | |||||||||||
\(\ds \) | \(=\) | \(\ds \frac {3 x} 8 + \frac 1 2 \paren {\frac {\sin 2 a x} {2 a} } + \frac 1 8 \paren {\frac {\sin 4 a x} {4 a} } + C\) | Primitive of $\cos a x$ | |||||||||||
\(\ds \) | \(=\) | \(\ds \frac {3 x} 8 + \frac {\sin 2 a x} {4 a} + \frac {\sin 4 a x} {32 a} + C\) | simplifying |
$\blacksquare$
Also see
Sources
- 1968: Murray R. Spiegel: Mathematical Handbook of Formulas and Tables ... (previous) ... (next): $\S 14$: Integrals involving $\cos a x$: $14.380$