Trig Substitution Homework: Solving \int{cos^4 6x sin^3 6x dx}

In summary, the given integral of cos^4 6x sin^3 6x dx can be simplified using the power-reducing formula and the pythagorean identity to eventually make use of a simple substitution, leading to a solution.
  • #1
duki
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Homework Statement



[tex]\int{cos^4 6x sin^3 6x dx}[/tex]

Homework Equations



The Attempt at a Solution



I've gotten this far but now I'm stuck:

[tex]\int{cos^4 6x sin 6x sin^2 6x dx} = \int{cos^4 6x}*(\frac{1-cos 12x}{2})sin 6x dx[/tex]
 
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  • #2
From you having the integrand as this: cos^4 6x sin^3 6x dx

Use a substitution u = cos^4 6x
 
  • #3
I don't understand exactly. Are you saying use substitution after using the power-reducing formula? Or do I not use power-reducing at all here? I briefly saw him explaining it to one person, and he mentioned something about reducing it from sin^3 to sin^2... I could have misunderstood though.
 
  • #4
Sorry, that substitution was wrong. Tomorrow when I'm more awake (it's 3:40 AM here) I'll have another look at it.
 
  • #5
duki said:
I don't understand exactly. Are you saying use substitution after using the power-reducing formula? Or do I not use power-reducing at all here? I briefly saw him explaining it to one person, and he mentioned something about reducing it from sin^3 to sin^2... I could have misunderstood though.

Use the fact that [tex]sin^3 (6x) =sin^2 (6x) . sin(6x)[/tex]. Then perhaps make use of the pythagorean identity, and you will be 1 simple substitution away from a solution :smile:
 

FAQ: Trig Substitution Homework: Solving \int{cos^4 6x sin^3 6x dx}

What is Trig Substitution?

Trig substitution is a technique used in calculus to simplify integrals that involve trigonometric functions. It involves substituting trigonometric expressions in place of other algebraic expressions in order to make the integral easier to solve.

Why is Trig Substitution used?

Trig substitution is used to simplify integrals that involve trigonometric functions, making them easier to solve. It is also used to solve integrals that cannot be solved using other techniques such as u-substitution or integration by parts.

How do I solve \int{cos^4 6x sin^3 6x dx} using Trig Substitution?

To solve this integral using trig substitution, we first use the trigonometric identity cos^2x = (1+cos2x)/2 to rewrite cos^4 6x as [(1+cos12x)/2]^2. Similarly, we can use the identity sin^2x = (1-cos2x)/2 to rewrite sin^3 6x as [(1-cos12x)/2]^3. Then, we can substitute u = cos12x and du = -6sin12x dx, which simplifies the integral to \int{(1+u)^2(1-u)^3(-du)/12}. This can be expanded and integrated using the power rule.

What are the common Trig Substitution identities?

The common Trig Substitution identities include sin^2x = (1-cos2x)/2, cos^2x = (1+cos2x)/2, tan^2x = (sec^2x-1)/2, sec^2x = (tan^2x+1)/2, and csc^2x = (cot^2x+1)/2. These identities can be used to simplify integrals involving trigonometric functions.

Are there any tips for solving integrals using Trig Substitution?

Yes, here are some tips for solving integrals using Trig Substitution: 1) Look for trigonometric identities that can be used to simplify the integral, 2) Substitute u = trigonometric function and du = differential of the trigonometric function, 3) Use algebraic manipulations to simplify the integral, and 4) Make sure to substitute back for the original variable at the end of the integral.

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