Power Reduction Formulas

Concept of Power Reduction

Power reduction refers to the method of reducing high powers of trigonometric functions (such as $\sin^n x$ , $\cos^n x$ ) to lower powers or single trigonometric functions. Power reduction formulas are an important part of trigonometric identities and have important applications in integral calculations, series expansions, and signal processing.

Basic Power Reduction Formulas

Square Power Reduction Formulas

The most basic power reduction formulas reduce square terms to single terms:

Sine Square Power Reduction Formula

\sin^2 x = \frac{1 - \cos 2x}{2}

Cosine Square Power Reduction Formula

\cos^2 x = \frac{1 + \cos 2x}{2}

Derivation Process

These formulas can be derived from the double angle formulas for cosine:

Derivation of Sine Square Formula:

From the cosine double angle formula: $\cos 2x = \cos^2 x - \sin^2 x$
Combined with the basic identity: $\sin^2 x + \cos^2 x = 1$ , i.e., $\cos^2 x = 1 - \sin^2 x$
Substitute into the double angle formula: $\cos 2x = (1 - \sin^2 x) - \sin^2 x = 1 - 2\sin^2 x$
Rearrange to get: $2\sin^2 x = 1 - \cos 2x$
Therefore: $\sin^2 x = \frac{1 - \cos 2x}{2}$

Derivation of Cosine Square Formula:

From the cosine double angle formula: $\cos 2x = \cos^2 x - \sin^2 x$
Combined with the basic identity: $\sin^2 x = 1 - \cos^2 x$
Substitute into the double angle formula: $\cos 2x = \cos^2 x - (1 - \cos^2 x) = 2\cos^2 x - 1$
Rearrange to get: $2\cos^2 x = 1 + \cos 2x$
Therefore: $\cos^2 x = \frac{1 + \cos 2x}{2}$

Cubic Power Reduction Formulas

For third powers, there are also corresponding power reduction formulas:

Sine Cubic Power Reduction Formula

\sin^3 x = \frac{3\sin x - \sin 3x}{4}

Cosine Cubic Power Reduction Formula

\cos^3 x = \frac{3\cos x + \cos 3x}{4}

Derivation Process

Derivation of Sine Cubic Formula:

Using the triple angle formula and identity transformations:

Triple angle formula: $\sin 3x = 3\sin x - 4\sin^3 x$
Rearrange to get: $4\sin^3 x = 3\sin x - \sin 3x$
Therefore: $\sin^3 x = \frac{3\sin x - \sin 3x}{4}$

Derivation of Cosine Cubic Formula:

Triple angle formula: $\cos 3x = 4\cos^3 x - 3\cos x$
Rearrange to get: $4\cos^3 x = 3\cos x + \cos 3x$
Therefore: $\cos^3 x = \frac{3\cos x + \cos 3x}{4}$

General Power Reduction Formulas

For higher powers, recursive relationships or binomial expansions can be used. For nth powers:

定理 1

Let $n$ be a positive integer, then:

When $n = 2k$ ( $k$ is a positive integer), $\sin^{2k} x$ and $\cos^{2k} x$ can be reduced to linear combinations of $\cos 2x, \cos 4x, \ldots, \cos 2kx$
When $n = 2k+1$ ( $k$ is a positive integer), $\sin^{2k+1} x$ can be reduced to linear combinations of $\sin x, \sin 3x, \ldots, \sin(2k+1)x$ , and $\cos^{2k+1} x$ can be reduced to linear combinations of $\cos x, \cos 3x, \ldots, \cos(2k+1)x$

Common Power Reduction Formulas Summary

Fourth Power Reduction Formulas

\begin{aligned} \sin^4 x &= \frac{3 - 4\cos 2x + \cos 4x}{8} \\ \cos^4 x &= \frac{3 + 4\cos 2x + \cos 4x}{8} \end{aligned}

Fifth Power Reduction Formulas

\begin{aligned} \sin^5 x &= \frac{10\sin x - 5\sin 3x + \sin 5x}{16} \\ \cos^5 x &= \frac{10\cos x + 5\cos 3x + \cos 5x}{16} \end{aligned}

Applications

Power reduction formulas have wide applications in mathematics and engineering:

1. Integral Calculations

Power reduction formulas can convert integrals of high-power trigonometric functions into integrals of single terms or low-power terms, simplifying the calculation process.

Example:

\int \sin^2 x \, dx = \int \frac{1 - \cos 2x}{2} \, dx = \frac{x}{2} - \frac{\sin 2x}{4} + C

2. Fourier Series

In Fourier series expansion, power reduction formulas can help expand complex functions into simple trigonometric series.

3. Signal Processing

In the field of signal processing, power reduction formulas are used for signal modulation, filtering, and other operations.

4. Proof of Trigonometric Identities

Power reduction formulas are important tools for proving other trigonometric identities.

Exercises

Exercise1

Reduce $\sin^2 x \cos^2 x$ using power reduction formulas and simplify.

Reference Answer (3 个标签)

power reduction formulas trigonometric identities product simplification

Problem-solving approach: Use square power reduction formulas to reduce $\sin^2 x$ and $\cos^2 x$ respectively, then multiply and simplify.

Detailed steps:

From the power reduction formulas:
$\sin^2 x = \frac{1 - \cos 2x}{2}, \quad \cos^2 x = \frac{1 + \cos 2x}{2}$
Multiply to get:
$\sin^2 x \cos^2 x = \frac{(1 - \cos 2x)(1 + \cos 2x)}{4} = \frac{1 - \cos^2 2x}{4}$
Use the power reduction formula again: $\cos^2 2x = \frac{1 + \cos 4x}{2}$
Substitute to get:
$\sin^2 x \cos^2 x = \frac{1 - \frac{1 + \cos 4x}{2}}{4} = \frac{2 - 1 - \cos 4x}{8} = \frac{1 - \cos 4x}{8}$

Answer: $\sin^2 x \cos^2 x = \frac{1 - \cos 4x}{8}$

Exercise2

Use power reduction formulas to calculate $\int_0^{\pi} \sin^4 x \, dx$ .

Reference Answer (3 个标签)

power reduction formulas definite integral trigonometric function integration

Problem-solving approach: First expand $\sin^4 x$ using power reduction formulas, then integrate term by term.

Detailed steps:

Use the power reduction formula:
$\sin^4 x = \frac{3 - 4\cos 2x + \cos 4x}{8}$
Integrate term by term:
$\begin{aligned} \int_0^{\pi} \sin^4 x \, dx &= \int_0^{\pi} \frac{3 - 4\cos 2x + \cos 4x}{8} \, dx \\ &= \frac{1}{8} \left[ 3x - 2\sin 2x + \frac{\sin 4x}{4} \right]_0^{\pi} \\ &= \frac{1}{8} \left[ 3\pi - 0 - (0 - 0) \right] \\ &= \frac{3\pi}{8} \end{aligned}$

Answer: $\frac{3\pi}{8}$

Exercise3

Prove: $\sin^6 x + \cos^6 x = \frac{5 + 3\cos 4x}{8}$ .

Reference Answer (3 个标签)

power reduction formulas trigonometric identities identity proof

Problem-solving approach: Use power reduction formulas and the sum of cubes formula.

Detailed steps:

Use the sum of cubes formula: $a^3 + b^3 = (a + b)(a^2 - ab + b^2)$
Let $a = \sin^2 x$ , $b = \cos^2 x$ , then:
$\sin^6 x + \cos^6 x = (\sin^2 x)^3 + (\cos^2 x)^3 = (\sin^2 x + \cos^2 x)(\sin^4 x - \sin^2 x \cos^2 x + \cos^4 x)$
Since $\sin^2 x + \cos^2 x = 1$ , so:
$\sin^6 x + \cos^6 x = \sin^4 x - \sin^2 x \cos^2 x + \cos^4 x$
Use power reduction formulas:
- $\sin^4 x = \frac{3 - 4\cos 2x + \cos 4x}{8}$
- $\cos^4 x = \frac{3 + 4\cos 2x + \cos 4x}{8}$
- $\sin^2 x \cos^2 x = \frac{1 - \cos 4x}{8}$ (from Exercise 1)
Substitute and calculate:
$\begin{aligned} \sin^6 x + \cos^6 x &= \frac{3 - 4\cos 2x + \cos 4x}{8} + \frac{3 + 4\cos 2x + \cos 4x}{8} - \frac{1 - \cos 4x}{8} \\ &= \frac{3 - 4\cos 2x + \cos 4x + 3 + 4\cos 2x + \cos 4x - 1 + \cos 4x}{8} \\ &= \frac{5 + 3\cos 4x}{8} \end{aligned}$

Answer: Proof completed.

Exercise4

Find the maximum and minimum values of the function $f(x) = \sin^4 x + \cos^4 x$ .

Reference Answer (3 个标签)

power reduction formulas function properties maximum and minimum values

Problem-solving approach: Simplify the function using power reduction formulas, then analyze the range.

Detailed steps:

Use power reduction formulas:
$\sin^4 x + \cos^4 x = \frac{3 - 4\cos 2x + \cos 4x}{8} + \frac{3 + 4\cos 2x + \cos 4x}{8} = \frac{6 + 2\cos 4x}{8} = \frac{3 + \cos 4x}{4}$
Since $\cos 4x \in [-1, 1]$ :
- When $\cos 4x = 1$ , $f(x) = \frac{3 + 1}{4} = 1$
- When $\cos 4x = -1$ , $f(x) = \frac{3 - 1}{4} = \frac{1}{2}$

Answer:

Maximum value: $1$
Minimum value: $\frac{1}{2}$

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