Increase and Decrease of a function question

In summary: However, it is not clear what the conversation is about. Can you please provide a summary of the conversation?In summary, the conversation is about determining whether the function f(x) = x - cosx is increasing or decreasing between the interval of 0 ≤ x ≤ 2π. The person asking the question has taken the derivative and is not sure how to proceed. Another person suggests looking at the range of sin(x) to determine the behavior of the function. The original problem is not solved, but the double angle identity for sine is mentioned as a possible solution.
  • #1
appplejack
43
0

Homework Statement


f(x)=x — cosx, 0 ≤ x ≤ 2∏


Homework Equations





The Attempt at a Solution



I took the derivative of the function: f'(x)=1+ sinx, 0 ≤ x ≤ 2∏
I don't know how to determine whether it is increasing or decreasing between the interval given. What should I do?
 
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  • #2
appplejack said:

Homework Statement


f(x)=x — cosx, 0 ≤ x ≤ 2∏

Homework Equations



The Attempt at a Solution



I took the derivative of the function: f'(x)=1+ sinx, 0 ≤ x ≤ 2∏
I don't know how to determine whether it is increasing or decreasing between the interval given. What should I do?
What do you know about the range of sin(x) ?
 
  • #3
The Range of sin(x) is -1≤ x ≤ 1?

I have an additional question:

I saw this in my textbook and don't understand why this is true.

f (x) = -2sin2x — 2sinx = -2sinx(2cosx+1) <— how is this true?
Thanks
 
  • #4
appplejack said:
The Range of sin(x) is -1≤ x ≤ 1?

I have an additional question:

I saw this in my textbook and don't understand why this is true.

f (x) = -2sin2x — 2sinx = -2sinx(2cosx+1) <— how is this true?
Thanks
So... Have you solve the original problem?

As for

f (x) = -2sin2x — 2sinx
= -2sinx(2cosx+1)​
The double angle identity for sine is: sin(2x) =2sin(x)cos(x)

Therefore: -2(2sin(x)cos(x)+sin(x)) =  ? 
 
  • #5
No. I drew the graph and it doesn't always increase but the answer says it increases from [0,2∏] How does knowing the range of sine solve help?

Actually, I've never learned "The double angle identity". I guess I have to know this then.
 
  • #6
Well, you got the derivative: f'(x)=1+ sin(x).

-1 ≤ sin(x) ≤ 1

Therefore:

0 ≤ 1+ sin(x) ≤ 2

So f' is positive except where sin(x) = -1, correct ?

Take it from there.

(It's a little tricky to figure out the behavior where f' is zero.)
 
  • #7
Thank you SammyS.
 

Related to Increase and Decrease of a function question

1. What is the definition of a function?

A function is a mathematical relationship between two variables, where each input (or independent variable) has only one output (or dependent variable).

2. How do you determine if a function is increasing or decreasing?

A function is increasing if its output (or dependent variable) increases as the input (or independent variable) increases. A function is decreasing if its output decreases as the input increases.

3. What is the difference between a local maximum and a global maximum?

A local maximum is the highest point on a specific section of a function, while a global maximum is the highest point on the entire function. A global maximum can also be referred to as an absolute maximum.

4. How do you find the critical points of a function?

Critical points are the points where the derivative of a function is equal to zero or undefined. To find these points, you can set the derivative of the function equal to zero and solve for the input value(s).

5. How can you use the first derivative test to determine if a point is a local maximum or minimum?

The first derivative test states that if the derivative of a function is positive at a point, then the function is increasing at that point and the point is a local minimum. If the derivative is negative at a point, then the function is decreasing at that point and the point is a local maximum.

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