A trigonometric substitution problem

In summary, the problem involves solving the integral of sin3cos2x with the use of factoring and substitution. The attempt at solving the problem involved assigning u=cosx and du=-sinx, but using u=sin3x did not work due to an incorrect derivative. The correct derivative is 3sin(x)^2*cos(x).
  • #1
celeramo
9
0

Homework Statement



[tex]\int[/tex]sin3cos2xdx



The Attempt at a Solution



I've successfully solved this problem by factoring out 1 sinx and changing the sin2x to (1-cos2x then assigning u=cosx and du=-sinx and so on.

What I'm wondering is why does letting u=sin3x in the original integral not work. Then du=3cos2xdx and there is a cos2x in the original integral. Why does this fail? Please and thank you :)
 
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  • #2
Well, the reason that doesn't work is that that is not the correct derivative. the derivative is 3sin(x)^2*cos(x) :) remember taking the derivative of v^3 is 3v^2*dv (where I let v=sin(x))
 
  • #3
Thanks very much. I don't know what I was thinking
 

FAQ: A trigonometric substitution problem

What is a trigonometric substitution problem?

A trigonometric substitution problem is a type of calculus problem that involves using trigonometric functions to simplify an integral or equation. It is commonly used to solve integrals that involve radicals or expressions with powers of trigonometric functions.

When should I use trigonometric substitution?

Trigonometric substitution is typically used when you have an integral that contains terms like √(a²-x²) or √(x²-a²) or terms that involve powers of trigonometric functions such as sin^n(x) or cos^n(x). It allows you to simplify the integral and make it easier to solve.

How do I solve a trigonometric substitution problem?

To solve a trigonometric substitution problem, you need to identify the appropriate substitution to use based on the form of the integral. Then, you can substitute the appropriate trigonometric function and use trigonometric identities to simplify the integral. Once simplified, you can use standard integration techniques to find the solution.

What are some common trigonometric substitutions?

Some common trigonometric substitutions include using sin(x) or cos(x) for √(a²-x²), tan(x) for √(x²-a²), sec(x) for x√(x²-a²), and cot(x) for x√(a²-x²). These substitutions are chosen based on the form of the integral and can help simplify the problem.

What are some tips for solving trigonometric substitution problems?

Some tips for solving trigonometric substitution problems include carefully analyzing the integral to determine the appropriate substitution to use, being familiar with trigonometric identities to simplify the integral, and practicing with different types of problems to become more comfortable with the process.

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