6) Prove cos4x -sin+x= cos 2x

Calculus: Early Transcendentals
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Author:James Stewart
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Chapter1: Functions And Models
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Problem 1RCC: (a) What is a function? What are its domain and range? (b) What is the graph of a function? (c) How...
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**Problem 6: Prove the Trigonometric Identity**

Prove that:

\[ \cos^4 x - \sin^4 x = \cos 2x \]

This is a trigonometric identity that demonstrates the relationship between the fourth powers of sine and cosine functions and the double angle cosine. To prove this identity, utilize trigonometric identities and algebraic manipulation. 

**Solution Outline:**

1. Start with the left side of the equation, \( \cos^4 x - \sin^4 x \).
2. Use the difference of squares identity: 

\[ a^2 - b^2 = (a - b)(a + b) \]

   Thus, express \( \cos^4 x - \sin^4 x \) as:

   \[ (\cos^2 x - \sin^2 x)(\cos^2 x + \sin^2 x) \]

3. Recall the Pythagorean identity:

   \[ \cos^2 x + \sin^2 x = 1 \]

   Substitute this into the expression:

   \[ (\cos^2 x - \sin^2 x)(1) = \cos^2 x - \sin^2 x \]

4. Use the double angle identity for cosine:

   \[ \cos 2x = \cos^2 x - \sin^2 x \]

   Conclude that:

   \[ \cos^4 x - \sin^4 x = \cos 2x \]

This demonstrates the correctness of the identity through algebraic manipulation and application of known trigonometric identities.
Transcribed Image Text:**Problem 6: Prove the Trigonometric Identity** Prove that: \[ \cos^4 x - \sin^4 x = \cos 2x \] This is a trigonometric identity that demonstrates the relationship between the fourth powers of sine and cosine functions and the double angle cosine. To prove this identity, utilize trigonometric identities and algebraic manipulation. **Solution Outline:** 1. Start with the left side of the equation, \( \cos^4 x - \sin^4 x \). 2. Use the difference of squares identity: \[ a^2 - b^2 = (a - b)(a + b) \] Thus, express \( \cos^4 x - \sin^4 x \) as: \[ (\cos^2 x - \sin^2 x)(\cos^2 x + \sin^2 x) \] 3. Recall the Pythagorean identity: \[ \cos^2 x + \sin^2 x = 1 \] Substitute this into the expression: \[ (\cos^2 x - \sin^2 x)(1) = \cos^2 x - \sin^2 x \] 4. Use the double angle identity for cosine: \[ \cos 2x = \cos^2 x - \sin^2 x \] Conclude that: \[ \cos^4 x - \sin^4 x = \cos 2x \] This demonstrates the correctness of the identity through algebraic manipulation and application of known trigonometric identities.
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