Trigonometric Identity Verification | Simplifying sin(4x) and Solving for x

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In summary, using the double angle and half angle identities, we can simplify the given equation to sin(4x) = 2sin(2x)cos(2x). By further simplifying using the Pythagorean and double angle identities, we can show that this is equal to 8sin(x)cos3(x) - 4sin(x)cos(x).
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themadhatter1
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Homework Statement



sin(4x) = 8cos3(x)sin(x)-4sin(x)cos(x)

Homework Equations



All trigonometric identities

The Attempt at a Solution



I can simplify the right side using the double angle identity to:

sin(4x) = 4sin(2x)cos2(x)-2sin(2x)

However, now I'm not sure what to do. Did I take a step in the wrong direction?
 
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  • #2
Never mind, I found the solution myself, here is my process. I was on the right track.

sin(4x) = 4sin(2x)cos2(x)-2sin(2x)

Pythagorean Identity:

sin(4x) = 4sin(2x)(1-sin2x)-2sin(2x)

FOIL:

sin(4x) = 4sin(2x)-4sin(2x)sin2(x)-2sin(2x)

Half Angle Identity:

sin(4x) = 4sin(2x)-4sin(2x)[(1-cos(2x)/2]-2sin(2x)

simplify:

sin(4x) = 4sin(2x)-[4sin(2x)+4sin(2x)cos(2x)]/(2)-2sin(2x)

simplify more:

sin(4x) = 4sin(2x)-2sin(2x)+2sin(2x)cos(2x)-2sin(2x)

sin(4x) = 4sin(2x)-4sin(2x)+2sin(2x)cos(2x)

sin(4x) = 2sin(2x)cos(2x)

Double Angle Identity:

sin(4x) = sin(4x)
 
  • #3
You can save yourself a lot of typing by working from the left side to the right.

sin(4x) = sin(2(2x)) = 2sin(2x)cos(2x)
= 4sin(x)cos(x)(cos2(x) - sin2(x))
= 4sin(x)cos(x)(2cos2(x) - 1)
= 8sin(x)cos3(x) - 4sin(x)cos(x)
QED
 

FAQ: Trigonometric Identity Verification | Simplifying sin(4x) and Solving for x

What is a trigonometric identity?

A trigonometric identity is an equation that is true for all values of the variables involved. It is used to simplify and manipulate trigonometric expressions.

What does it mean to simplify a trigonometric expression?

Simplifying a trigonometric expression means reducing it to its simplest form by using trigonometric identities and properties.

How do I simplify sin(4x)?

To simplify sin(4x), we can use the double angle identity for sine, which states that sin(2x) = 2sin(x)cos(x). Therefore, sin(4x) = 2sin(2x)cos(2x). We can then use the double angle identity for cosine, cos(2x) = 1 - 2sin^2(x), to further simplify the expression to sin(4x) = 4sin(x)cos(x) - 8sin^3(x)cos(x).

What is the process for solving for x in a trigonometric expression?

To solve for x in a trigonometric expression, we need to isolate the trigonometric function on one side of the equation and use algebraic techniques to solve for x. In the case of sin(4x), we can use the inverse sine function to isolate x: x = sin^-1(sin(4x)). However, it is important to remember that there may be multiple solutions for x depending on the values of the trigonometric function.

Can I use a calculator to verify trigonometric identities and solve for x?

Yes, you can use a calculator to verify trigonometric identities and solve for x. However, it is important to understand the steps and processes involved in order to use the calculator effectively and interpret the results correctly.

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