Finding derivative of Trig. Functions

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SUMMARY

The derivative of the function h(x) = (sin(2x))(cos(2x)) can be calculated using the product rule combined with the chain rule. The correct derivative is 2cos(4x), which aligns with the answer provided in the textbook. The error in the initial attempt stemmed from neglecting the chain rule when differentiating cos(2x), where the derivative is -2sin(2x). Additionally, utilizing trigonometric identities such as cos(2u) = cos²(u) - sin²(u) can simplify the process.

PREREQUISITES
  • Understanding of the product rule in calculus
  • Knowledge of the chain rule in differentiation
  • Familiarity with trigonometric identities
  • Basic proficiency in calculus, particularly derivatives of trigonometric functions
NEXT STEPS
  • Study the application of the product rule in calculus
  • Learn the chain rule and its implications in differentiation
  • Explore trigonometric identities and their uses in calculus
  • Practice finding derivatives of composite functions involving trigonometric functions
USEFUL FOR

Students studying calculus, particularly those focusing on derivatives of trigonometric functions, as well as educators seeking to clarify the application of differentiation rules.

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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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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)
 

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