Proving cos^4(x)= 3/8+1/2(cos2x)+1/8(cos4x)

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In summary, the student attempted to change the cos2x to 2cos^2(x)-1 in order to solve the homework equation, but got stuck. They then multiplied 2cos(4x) by 2 to get the cos4x, but forgot the 2x.
  • #1
banfill_89
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Homework Statement



prove that cos^4(x)= 3/8 +1/2(cos2x)+1/8(cos4x)

Homework Equations





The Attempt at a Solution



i attempted to change the cos2x to 2cos^2(x)-1...then made all them over 8...and now I am stuck...another question...if i multiply 2cos(4x) by 2, do i multiply the 2 and the 4x? or just he 4x?
 
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  • #2
Well...Use the fact that cos^2(x) = (1 + cos(2x))/2 ...Then square both sides...you will then be pretty close...(that is..you will have cos^4(x) = some mess)..you will just need to use that above formula again to reduce the (cos^2(2x)) you get after squaring the right hand side of the equation...make sure you realize that when plugging (2x) into the left hand side...you will need to make the same plug in in the right...after that...the problem should unfold nicely... You just needed a different formula..
 
  • #3
do i isolate the cos(2x) in cos^2(x) = (1 + cos(2x))/2 and sun that in?
 
  • #4
.. I don't know what you mean.. With this method...that is starting with the formula "cos^2(x) = (1 + cos(2x))/2", after squaring and showing that the left hand side is indeed Cos^4(x), your goal is to git rid of every square you have on the right hand side of that...and it turns out, there is only 1...so you use the formula again, but make sure you keep track of that doubled variable...
 
  • #5
quick...and dumb question...

i get 3+cos2x+2cos2x all over 8...which id close except the cos2x should be a cos4x
 
  • #6
Ha! Try again...Your are making mistakes in your arithmetic.. Just take it nice and slow... The above isn't as close as you might think...because not only should a cos2x be a 4x...but 2cos2x divided by 8...isnt 1/2cos2x...
 
  • #7
lmao...il try it again slow
 
  • #8
ha...tell me what you get
 
  • #9
ahhhh i got it lol...when i was working out the cos^2(2x), i wrote it originally as 1+cos2x over 2, instead of 1+cos4x over 2...is that right now?
 
  • #10
I told you to watch out for that 2x! ><
 

FAQ: Proving cos^4(x)= 3/8+1/2(cos2x)+1/8(cos4x)

How can I prove the equation cos^4(x) = 3/8 + 1/2(cos2x) + 1/8(cos4x)?

There are multiple ways to prove this equation. One approach is to use the double angle and power-reducing identities for cosine to simplify the left side of the equation and show that it is equal to the right side. Another approach is to use a graphing calculator to graph both sides and show that they are identical.

What is the significance of the constant terms 3/8, 1/2, and 1/8 in the equation?

The constant terms represent the coefficients of the various powers of cosine in the expanded form of the left side of the equation. They are important because they help to simplify and manipulate the equation to prove its validity.

Can this equation be used in real-world applications?

Yes, this equation can be used in various fields such as physics, engineering, and astronomy. Cosine is a fundamental trigonometric function that is used to model periodic phenomena, and this equation can help in solving problems related to such phenomena.

Is there a specific range of values for x in which this equation holds true?

Yes, this equation holds true for all values of x in the domain of cosine, which is all real numbers. However, if the equation is being used in a real-world application, the values of x may be limited by the context of the problem.

Are there any alternative forms of this equation?

Yes, there are alternative forms of this equation that involve different trigonometric functions or different powers of cosine. For example, cos^4(x) can be expressed as (1/2 + 1/2cos2x)^2 or (1/4 + 1/2cos2x + 1/4cos4x). The form of the equation used may depend on the specific problem or application it is being used for.

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