Finding derivative of Trig. Functions

In summary, the conversation discusses a problem involving the function h(x) = (sin2x)(cos2x) and the attempt to use the product rule to find its derivative. However, the solution does not match the answer given in the book. It is suggested that the error may be due to forgetting the chain rule when using the product rule. The correct answer is 2cos4x, which can also be found using the double angle formula for sin.
  • #1
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Homework Statement



h(x) = (sin2x)(cos2x)

Homework Equations



I attempted to use the product rule:

(sin2x)(-sin2x)+(cos2x)(cos2x) = cos22x-sin22x

The Attempt at a Solution



The book has the answer at 2cos4x, which I obviously didn't get. I've retraced my steps and while unsure of trig. functions, cannot find where I made my error.
 
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  • #2
You have forgotten the chain rule when you did the product rule. The derivative of cos(2x) will be -2sin(2x) [Note the 2 in front, from the chain rule].
Other than that, your answer is the same as the book, based on the identity that cos(2u)=cos^2(u)-sin^2(u)
(Alternatively you could have used the double angle formula for sin to rewrite your original function, and then found the derivative)
 

1. What is the definition of a derivative?

The derivative of a function is the rate of change of that function at a specific point. In other words, it represents the slope of the tangent line at that point.

2. How do you find the derivative of a trigonometric function?

To find the derivative of a trigonometric function, you can use the chain rule or the product rule, depending on the form of the function. For example, the derivative of sin(x) can be found using the chain rule as cos(x), while the derivative of cos(x)sin(x) would require the product rule.

3. What is the derivative of sine and cosine?

The derivative of sine is cosine, and the derivative of cosine is negative sine. This can be remembered using the mnemonic "COsine is NEgative sine."

4. Can the chain rule be applied to all trigonometric functions?

Yes, the chain rule can be applied to all trigonometric functions, as well as any other type of function. It is a general rule for finding derivatives of composite functions.

5. Why is it important to find the derivative of a trigonometric function?

Finding the derivative of a trigonometric function is important because it allows us to understand how the function is changing at a given point. This is useful in many applications, such as physics, engineering, and economics, where rates of change play a crucial role.

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