Derivative of a Sine Function Problem Confusion

In summary: So it's much better to use the chain rule from the start!In summary, the derivative of f(x) = sin²(3-x) is -2sin(3-x)cos(3-x) and this can be found using the chain rule.
  • #1
Loppyfoot
194
0

Homework Statement



If f(x) = sin²(3-x), then f ' (0) = ____ .

A. -2cos3
B. -2sin3cos3
C. 6cos3
D. 2sin3cos3
E. 6sin3cos3


Homework Equations



Derivative of Sinx= cosx.


The Attempt at a Solution



I cannot seem to figure out what the derivative would be with this function.

sin(3-x) * sin(3-x) = Cos(3-x) * Cos(3-x), then plug in 0 for x?

Is there a simpler way using "u" substitution?

Thanks!
 
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  • #2
Loppyfoot said:

Homework Statement



If f(x) = sin²(3-x), then f ' (0) = ____ .

A. -2cos3
B. -2sin3cos3
C. 6cos3
D. 2sin3cos3
E. 6sin3cos3


Homework Equations



Derivative of Sinx= cosx.


The Attempt at a Solution



I cannot seem to figure out what the derivative would be with this function.
What are you doing in the next line? Does it have anything to do with calculus? I don't see any indication that it does.
Loppyfoot said:
sin(3-x) * sin(3-x) = Cos(3-x) * Cos(3-x), then plug in 0 for x?


Is there a simpler way using "u" substitution?

Thanks!
Do you know about the chain rule? That's what you need here. Your function could be written this way f(x) = [sin(3 - x)]2.
 
  • #3
so, would [ sin(3-x)²] = 2[sin(3-x)] * cos(3-x) *-1?

I think what is confusing me is the square of the function.
 
  • #4
Almost. You're omitting the fact that your taking a derivative.
d/dx[ sin(3-x)²] = 2sin(3-x) * cos(3-x) *(-1) = -2sin(3 - x) cos(3 - x)
 
  • #5
Right. Thank you Sir. I know on the AP Calculus exam, the graders take off for not writing limit notation, derivative notation, etc. Thank you for your help, and I definitely understand this chain rule of trigononemtric functions much better.
 
  • #7
Loppyfoot said:
so, would [ sin(3-x)²] = 2[sin(3-x)] * cos(3-x) *-1?

I think what is confusing me is the square of the function.
Yes, that is correct. That is using the "chain rule".

And note that your earlier "sin(3-x) * sin(3-x) = Cos(3-x) * Cos(3-x)" was wrong for a variety of reasons:
1) You really mean (sin(3-x)*sin(3-x))' on the left. Don't write "=" for things that are not equal!

2) You haven't multiplied by the derivative of 3- x as you should.

3) Most importantly, the derivative of f(x)*f(x) is NOT " f'(x)*f'(x)". It is, by the product rule, f'(x)*f(x)+ f(x)*f'(x) which is equal to 2f(x)f'(x), exactly what applying the chain rule to f'(x)^2 would give you.
 

What is the derivative of a sine function?

The derivative of a sine function is the cosine function. This means that if the original function is f(x) = sin(x), then its derivative is f'(x) = cos(x).

How do I find the derivative of a sine function?

To find the derivative of a sine function, you can use the basic rules of differentiation. First, you need to rewrite the sine function in the form of f(x) = sin(x). Then, use the formula f'(x) = cos(x) to find the derivative.

Why is it important to find the derivative of a sine function?

Finding the derivative of a sine function is important because it allows us to understand the rate of change of the function at any given point. This is useful in many real-world applications, such as physics and engineering.

What is the relationship between the derivative of a sine function and its graph?

The derivative of a sine function is related to its graph in that the derivative gives us the slope of the function at any given point. This means that the derivative tells us how steep or flat the graph is at a specific point.

How can I use the derivative of a sine function to solve problems?

The derivative of a sine function can be used to solve problems involving rates of change, optimization, and related rates. By understanding the rate of change of the function, we can determine the maximum or minimum values, as well as how the function is changing over time.

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