Proving Trig Ident: sin(4s)/4 = cos^3(s)*Sin(s) - sin^3(s)*cos(s)

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In summary, the conversation discusses the simplification of the expression sin(4s)/4 into 2*sin(2s)*cos(2s)/4. The book also shows the use of the identity \cos(2s) \equiv \cos^2(s) - \sin^2(s) to simplify the expression further. The person also asks for clarification on why the expression is not 2*sin(2s)*(cos^2 - Sin^2)4. The expert explains that the book simply substituted in the identities for \sin(2s) and \cos(2s) to simplify the expression.
  • #1
Miike012
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Homework Statement


sin(4s)/4 = cos^3(s)*Sin(s) - sin^3(s)*cos(s)

In the book they did...
2*sin(2s)*cos(2s)/4
= 2*2*sin(s)*cos(s)/4 *(cos^2(s) - sin^2(s))
(I understand everything up until they multiplyed the 2*2*sin(s)*cos(s)/4 expression by cos^2(s) - sin^2(s)...
where did cos^2(s) - sin^2(s) come from?
 
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  • #2
[tex]\cos(2s) \equiv \cos^2(s) - \sin^2(s)[/tex]
 
  • #3
Yes that is true... but then why isn't the expression
2*sin(2s)*(cos^2 - Sin^2)4
?
 
  • #4
What they did was:
[tex]\frac{2\sin(2s)\cos(2s)}{4} = \frac{2\cdot \left(2\sin(s)\cos(s)\right)\left(\cos^2(s) - \sin^2(s)\right)}{4}[/tex]

which is simply substituting in [tex]2\sin(s)\cos(s)[/tex] for [tex]\sin(2s)[/tex], and [tex]\cos^2(s)-\sin^2(s)[/tex] for [tex]\cos(2s)[/tex].
 
  • #5
Thank you!
 
  • #6
Glad to help!
 

Related to Proving Trig Ident: sin(4s)/4 = cos^3(s)*Sin(s) - sin^3(s)*cos(s)

1. How do you prove the trig identity sin(4s)/4 = cos^3(s)*Sin(s) - sin^3(s)*cos(s)?

To prove this trig identity, we use the double angle formula for sine, which states that sin(2s) = 2sin(s)cos(s). We can then substitute this into the left side of the equation, giving us sin(4s)/4 = (2sin(2s)cos(2s))/4. We can further simplify this using the double angle formula for cosine, which states that cos(2s) = cos^2(s) - sin^2(s). Substituting this in and simplifying, we get sin(4s)/4 = cos^3(s)sin(s) - sin^3(s)cos(s), which matches the right side of the equation. Therefore, the identity is proved.

2. What is the purpose of proving trig identities?

Proving trig identities helps us to understand the relationships between different trigonometric functions and allows us to manipulate them to simplify complex expressions. This is useful in many areas of mathematics, physics, and engineering where trigonometric functions are commonly used.

3. Is there a specific method to follow when proving trig identities?

Yes, there are several methods that can be used to prove trig identities, including using algebraic manipulation, using the Pythagorean identities, using the sum and difference formulas, and using the double angle formulas. The method chosen will depend on the specific identity being proved and the individual's personal preference.

4. Can you prove a trig identity using a calculator?

No, calculators should not be used to prove trig identities. Proving identities requires mathematical reasoning and manipulation, not just plugging in values and checking if the equation holds true. Additionally, calculators can sometimes give incorrect results due to rounding errors.

5. Are there any tips for making proving trig identities easier?

Yes, there are a few tips that can make proving trig identities easier. These include being familiar with the various trigonometric identities, practicing regularly, using algebraic manipulation techniques, and breaking down complex expressions into smaller, more manageable parts. It is also helpful to have a good understanding of basic algebra and trigonometry concepts.

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