Finding horizontal tangents on an interval

In summary, the conversation discusses how to determine all values of x in the interval [-pi/2, pi/2] at which the graph has horizontal tangents. Two equations, f'(x)=9cos(x)-2sin(x) and f'(x)=-5csc(x)(5cot(x)-csc(x)), are provided and the conversation explores using inverse trigonometric functions to solve for the values of x.
  • #1
noelwolfe
3
0

Homework Statement



Determine all x in [-pi/2, pi/2] at which the graph has horizontal tangents.

Homework Equations



1.) f'(x)= 9cos(x)-2sin(x)
2.) f'(x)= -5csc(x) (5cot(x)-csc(x))

The Attempt at a Solution



1.) 9cos(x)=2sin(x)
9/2=sin(x)/cos(x)
9/2=tan(x)
and then I'm not sure what to do with 9/2?

2.) 5cot(x)=csc(x)
5= csc(x)/cot(x)
5=1/cos(x)
cos(x)=1/5
same problem here... what do I do with 1/5?
 
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  • #2
noelwolfe said:
9/2=tan(x)
and then I'm not sure what to do with 9/2?

You are looking for the values of x such that tan(x) = 9/2. Remember inverse trigonometric functions (i.e. sine / arcsine, cos / arccos, tan / arctan, etc.)?
 
  • #3
I'm sorry, I still don't know--I'm not aware of a "simple" solution to tan(x)=9/2. How would I go about finding the solution?
 
  • #4
Look up the arctan function (also called inverse tangent) and its uses.
 
  • #5
Got it. Thanks!
 

1. What is a horizontal tangent?

A horizontal tangent is a line that is parallel to the x-axis and touches a curve at a specific point. This means that the slope of the tangent line is equal to 0 at that point.

2. Why is it important to find horizontal tangents?

Finding horizontal tangents can help us identify critical points on a curve, which can be useful in optimization problems. It can also help us determine the concavity of a curve and identify points of inflection.

3. How do you find horizontal tangents on an interval?

To find horizontal tangents on an interval, we need to first find the derivative of the function. Then, we set the derivative equal to 0 and solve for the x-values. These x-values will be the points where the tangent line is horizontal. We can then check if these points fall within the given interval.

4. Can a function have more than one horizontal tangent on an interval?

Yes, a function can have more than one horizontal tangent on an interval. This can happen if the function has multiple points where the derivative is equal to 0 within the given interval.

5. Are there any other methods for finding horizontal tangents on an interval?

Yes, there are other methods such as using the second derivative test or graphing the function to visually identify points where the tangent line is horizontal. However, the most common and efficient method is to find the points where the derivative is equal to 0.

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