Differentiating a trig function

In summary, the question asks to find the derivative of y = sin(3x)cos(6x) and the student attempted to use the product rule. However, they made a mistake in their calculation by using 3 instead of 3x and forgetting the negative sign in front of sin(6x). The correct answer uses the trig identity cos(a)cos(b)-sin(a)sin(b) and simplifies to -3cos(9x)sin(6x) + 6sin(9x)cos(6x).
  • #1
steve snash
50
0

Homework Statement


Find dy/dx, given
y = sin( 3 x ) cos( 6 x )

Homework Equations


product rule= f'g+g'f

The Attempt at a Solution


I used the product rule and got this, but its supposedly wrong, what have i done wrong?((3*cos(x))*(6*cos(x)))+((6*sin(-x))*(3*sin(x)))
 
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  • #2
For starters, [itex]\frac{d}{dx}\sin(3x)=3\cos(3x)\neq3\cos(x)[/itex]
 
  • #3
Here's a general rule that may help you figure it out:

[tex]\frac{d}{dx}(sin(ax)) = a \times cos(ax)[/tex]
 
  • #4
cheers
 
  • #5
wtf, it still says its wrong,
((3*cos(3*x))*(6*cos(6*x)))+((6*sin(-6*x))*(3*sin(3*x)))
what am i still doing wrong?
 
  • #6
Where are the extra factors of 6 in your first term and 3 in your second term coming from?
 
  • #7
sorry i ment
((3*cos(3*x))*(cos(6*x)))+((6*sin(-6*x))*(sin(3*x)))
but its still wrong, can you simplify this?
 
  • #8
For starters, [itex]\sin(-6x)=-\sin(6x)[/itex]...can you think of a trig identity that involves something like [itex]\cos(a)\cos(b)-\sin(a)\sin(b)[/itex]?:wink:
 

Related to Differentiating a trig function

What is the process for differentiating a trig function?

The process for differentiating a trigonometric function involves using the rules of differentiation, such as the power rule or chain rule, to find the derivative of the function. This involves taking the derivative of each term in the function and simplifying the result.

What is the chain rule and how is it used to differentiate trig functions?

The chain rule is a rule of differentiation that allows us to find the derivative of a composite function. When differentiating a trigonometric function, the chain rule is used to differentiate the inner function first, then multiply it by the derivative of the outer function.

What are the common trig functions and their derivatives?

The most commonly used trigonometric functions are sine, cosine, tangent, cotangent, secant, and cosecant. Their derivatives are as follows:
- d/dx(sin x) = cos x
- d/dx(cos x) = -sin x
- d/dx(tan x) = sec^2 x
- d/dx(cot x) = -csc^2 x
- d/dx(sec x) = sec x * tan x
- d/dx(csc x) = -csc x * cot x

What is the derivative of the inverse trigonometric functions?

The derivatives of the inverse trigonometric functions are as follows:
- d/dx(arcsin x) = 1/sqrt(1-x^2)
- d/dx(arccos x) = -1/sqrt(1-x^2)
- d/dx(arctan x) = 1/(1+x^2)
- d/dx(arccot x) = -1/(1+x^2)
- d/dx(arcsec x) = 1/(x*sqrt(x^2-1))
- d/dx(arccsc x) = -1/(x*sqrt(x^2-1))

What are some common trig identities used in differentiating trig functions?

Some common trig identities used in differentiating trig functions include:
- sin^2 x + cos^2 x = 1
- tan x = sin x / cos x
- cot x = cos x / sin x
- sec x = 1 / cos x
- csc x = 1 / sin x
- d/dx(ln x) = 1/x

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