Understanding the Behavior of Trigonometric Functions on Specific Intervals

In summary, the statement "Therefore arctan(tan(x)/a) must increase continuously with x" is not accurate as the function increases only on certain intervals and is discontinuous at certain points.
  • #1
Miike012
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I have a question regarding what I highlighted in the paint doc.

How can they assume that the function is increasing?

As a matter of fact its not increasing on the entire interval from 0 to pi.
 

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  • #2
Miike012 said:
I have a question regarding what I highlighted in the paint doc.

How can they assume that the function is increasing?

As a matter of fact its not increasing on the entire interval from 0 to pi.

Well, clearly it isn't increasing. It isn't even continuous on [0,pi] if a=1/9. It is increasing where it is continuous as the derivative shows. There has got to be more context in the problem than you have shown. Where did you find this problem?
 
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  • #3
Dick said:
Well, clearly it isn't increasing. It isn't even continuous on [0,pi] if a=1/9. It is increasing where it is continuous as the derivative shows. There has got to be more context in the problem than you have shown. Where did you find this problem?

The paint document contains the entire paragraph.
 

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  • #4
The attempt to integrate through pi/2 this way looks doubtful. I would break the integration range at pi/4 etc.
##\int_{\frac{\pi}4}^{\frac{\pi}2}\frac{\sec^2(\theta).d\theta}{a^2+ \tan ^2(\theta)} = ## ##\int_0^{\frac{\pi}4}\frac{cosec^2(\theta).d\theta}{a^2+\cot^2(\theta)} = \int_0^{\frac{\pi}4}\frac{\sec^2(\theta).d\theta}{a^2\tan^2(\theta)+1} ##
 
  • #5
haruspex said:
The attempt to integrate through pi/2 this way looks doubtful. I would break the integration range at pi/4 etc.
##\int_{\frac{\pi}4}^{\frac{\pi}2}\frac{\sec^2(\theta).d\theta}{a^2+ \tan ^2(\theta)} = ## ##\int_0^{\frac{\pi}4}\frac{cosec^2(\theta).d\theta}{a^2+\cot^2(\theta)} = \int_0^{\frac{\pi}4}\frac{\sec^2(\theta).d\theta}{a^2\tan^2(\theta)+1} ##

I'm really not sure what the text Miike012 posted is getting at. But I would hope it's trying to say that if you are using the antiderivative arctan(tan(x)/a), you need to break the integration range at each point where that function is discontinuous. And arctan(tan(x)/a) is not the same thing as arctan(tan(x/a)) which is a problem with Miike012's original post.
 
  • #6
Dick said:
I'm really not sure what the text Miike012 posted is getting at. But I would hope it's trying to say that if you are using the antiderivative arctan(tan(x)/a), you need to break the integration range at each point where that function is discontinuous.
Agreed, but it didn't look to me as though the line taken in the attachment was helpful. Hence my proposed alternative.
And arctan(tan(x)/a) is not the same thing as arctan(tan(x/a)) which is a problem with Miike012's original post.
I don't see anywhere that arctan(tan(x/a)) is mentioned.
 
  • #7
haruspex said:
Agreed, but it didn't look to me as though the line taken in the attachment was helpful. Hence my proposed alternative.

I don't see anywhere that arctan(tan(x/a)) is mentioned.

I was trying to guess the sense of the question from the graphs posted. Since arctan(tan(x/9)) was graphed, I thought that might have something to do with what the OP thought the antiderivative was. It should have been arctan(tan(x)/9).
 
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  • #8
I don't care about anything else other than the statement made in the attachment which was..."Therefore arctan(tan(x)/a) must increase continuously with x." This statement can be found at the bottom of the attachment of post number 3.

Which was my original question in post number 1.
 
  • #9
Miike012 said:
I don't care about anything else other than the statement made in the attachment which was..."Therefore arctan(tan(x)/a) must increase continuously with x." This statement can be found at the bottom of the attachment of post number 3.

Which was my original question in post number 1.

Ok, fine. It doesn't. It increases from 0 to pi/2 if a is positive, then decreases discontinuously, then increases again from pi/2 to pi.
 
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FAQ: Understanding the Behavior of Trigonometric Functions on Specific Intervals

What is a trig equation?

A trigonometric equation involves one or more trigonometric functions and an unknown variable. The goal is to find the value of the variable that satisfies the equation.

How do you integrate a trig equation?

To integrate a trigonometric equation, you can use trigonometric identities and integration techniques such as substitution, integration by parts, or partial fractions.

What are the most common trigonometric identities used in integrating trig equations?

The most commonly used trigonometric identities in integrating trig equations are the Pythagorean identities, double angle identities, and half angle identities.

What are some tips for solving tricky trig equations?

Some tips for solving tricky trig equations include using symmetry to simplify the equation, substituting trigonometric expressions for the variable, and breaking the equation into smaller parts.

Why is it important to check your answer when integrating a trig equation?

It is important to check your answer when integrating a trig equation because trigonometric functions have multiple periodic solutions, so it is possible to get an incorrect answer that satisfies the equation. Checking your answer helps to ensure that you have found the correct solution.

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