How Can Double-Angle Identities Simplify Trigonometric Equations?

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SUMMARY

The discussion focuses on verifying the trigonometric identity \( \frac{1}{2} \sin(4x) = 2\sin x \cos x - 4\sin 3x \cos x \). The user attempts to use the sine subtraction formula \( \sin(A - B) = \sin A \cos B - \cos A \sin B \) but struggles with the manipulation of terms. The suggested approach involves rewriting \( \frac{1}{2} \sin(4x) \) as \( \frac{1}{2} \sin(2x + 2x) \) and expanding it using the addition formula, which simplifies the verification process.

PREREQUISITES
  • Understanding of trigonometric identities, specifically double-angle identities.
  • Familiarity with the sine addition formula: \( \sin(A + B) \).
  • Basic algebraic manipulation skills for simplifying trigonometric expressions.
  • Knowledge of sine and cosine functions and their properties.
NEXT STEPS
  • Study the derivation and application of double-angle identities in trigonometry.
  • Learn how to apply the sine addition formula in various trigonometric problems.
  • Practice verifying trigonometric identities with different approaches and techniques.
  • Explore the relationship between sine and cosine functions in the context of trigonometric equations.
USEFUL FOR

Students studying trigonometry, educators teaching trigonometric identities, and anyone looking to improve their skills in verifying trigonometric equations.

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Homework Statement


I've been having problems with a lot of identity problems lately, but I've found that I'm especially having issues with problems like this one;
Verify the identity,

1/2 Sin (4x) = 2SinxCosx-4Sin3xCosx


Homework Equations


My professor told me you use Sin(A-B)=SinACosB-CosASinB

The Attempt at a Solution


The best I could do was

1/2 Sin4x=2SinxCox-4Sin3xCos

1/2Sin4x=2(SinxCosx-2sin3)

1/2Sin4x=2Sin(x-4x)=-2Sin(3x)

I know that this is a horrible attempt at solving, but some help would be greatly appreciated.
 
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MaoRaygo said:

Homework Statement


I've been having problems with a lot of identity problems lately, but I've found that I'm especially having issues with problems like this one;
Verify the identity,

1/2 Sin (4x) = 2SinxCosx-4Sin3xCosx


Homework Equations


My professor told me you use Sin(A-B)=SinACosB-CosASinB

The Attempt at a Solution


The best I could do was

1/2 Sin4x=2SinxCox-4Sin3xCos

1/2Sin4x=2(SinxCosx-2sin3)

1/2Sin4x=2Sin(x-4x)=-2Sin(3x)

I know that this is a horrible attempt at solving, but some help would be greatly appreciated.

Start with the LHS and write \frac{1}{2} \sin(4x)\,\,\text{as}\,\, \frac{1}{2} \sin(2x +2x) and expand using the addition formula as your professor suggested.
 

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