Arctan limit (without L'Hopital's Rule)

In summary, the conversation discusses finding the limit of arctan(1+x)/(1-x) as x approaches positive infinity. The OP initially asks for confirmation on their answer of -1, which they obtained by using the ratio of the leading coefficients and taking the arctan of -1 in their calculator. The conversation then delves into the importance of using radians instead of degrees and the book's lack of answers for even-numbered problems. The final clarification is given that the correct answer should be -π/4 instead of -45, and the importance of using parentheses when inputting the equation into the calculator is emphasized.
  • #1
AlexandraMarie112
16
1

Homework Statement


Limx-->positive infty arctan(1+x)/(1-x)

Homework Equations

The Attempt at a Solution


I just need to know if my answer is right.

Knowing that when the leading coefficients of the x when its the same, then the answer is just the ratio. So it would be -1. Then in my calculator I put arctan -1 and it gave -45. Is this right?
 
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  • #2
AlexandraMarie112 said:

Homework Statement


Limx-->positive infty arctan(1+x)/(1-x)

Homework Equations

The Attempt at a Solution


I just need to know if my answer is right.

Knowing that when the leading coefficients of the x when its the same, then the answer is just the ratio. So it would be -1. Then in my calculator I put arctan -1 and it gave -45. Is this right?
In math problems, trig functions and their inverses are usually in radians. Your calculator is set to degree mode. The book's answer will almost certainly be in radians. Otherwise, your value of -1 is correct.
 
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  • #3
Mark44 said:
In math problems, trig functions and their inverses are usually in radians. Your calculator is set to degree mode. The book's answer will almost certainly be in radians. Otherwise, your value of -1 is correct.

The answer is not in the book. My book has a weird way of giving the answers. Only gives for odd numbered questions and this ones an even so I have no way of knowing if my answer is right.

So instead of -45 it would be -pi/4 ?
 
  • #4
AlexandraMarie112 said:

Homework Statement


Limx-->positive infty arctan(1+x)/(1-x)

Homework Equations

The Attempt at a Solution


I just need to know if my answer is right.

Knowing that when the leading coefficients of the x when its the same, then the answer is just the ratio. So it would be -1. Then in my calculator I put arctan -1 and it gave -45. Is this right?

Do you mean
$$\frac{\arctan(1+x)}{1-x}$$
or do you mean
$$\arctan \left( \frac{1+x}{1-x} \right)?$$
If you mean the latter you need parentheses, like this: arctan((1+x)/(1-x)) or arctan[(1-x)/(1+x)].
 
  • #5
AlexandraMarie112 said:
The answer is not in the book. My book has a weird way of giving the answers. Only gives for odd numbered questions and this ones an even so I have no way of knowing if my answer is right.
That's not at all unusual for math books. If the answer you get for an odd-numbered problem agrees with the book's answer, you can have some confidence that your work for a nearby even-numbered problem is also correct.

AlexandraMarie112 said:
So instead of -45 it would be -pi/4 ?
Right.

Ray Vickson said:
If you mean the latter you need parentheses, like this: arctan((1+x)/(1-x)) or arctan[(1-x)/(1+x)].
I agree completely. I assumed from the answer given by the OP, that the problem was as you show it here, Ray.
 
  • #6
Mark44 said:
That's not at all unusual for math books. If the answer you get for an odd-numbered problem agrees with the book's answer, you can have some confidence that your work for a nearby even-numbered problem is also correct.

Right.

I agree completely. I assumed from the answer given by the OP, that the problem was as you show it here, Ray.

I agree that we can try to guess the meaning by looking at what the OP has done; but the OP needs to realize that clearer communication is important.
 
  • #7
Ray Vickson said:
but the OP needs to realize that clearer communication is important.
Absolutely.
 
  • #8
Ray Vickson said:
I agree that we can try to guess the meaning by looking at what the OP has done; but the OP needs to realize that clearer communication is important.
Ray Vickson said:
Do you mean
$$\frac{\arctan(1+x)}{1-x}$$
or do you mean
$$\arctan \left( \frac{1+x}{1-x} \right)?$$
If you mean the latter you need parentheses, like this: arctan((1+x)/(1-x)) or arctan[(1-x)/(1+x)].

Its the second one , but in my original post that's exactly how I entered it.
 
  • #9
Ray Vickson said:
I agree that we can try to guess the meaning by looking at what the OP has done; but the OP needs to realize that clearer communication is important.
Ray Vickson said:
Do you mean
$$\frac{\arctan(1+x)}{1-x}$$
or do you mean
$$\arctan \left( \frac{1+x}{1-x} \right)?$$
If you mean the latter you need parentheses, like this: arctan((1+x)/(1-x)) or arctan[(1-x)/(1+x)].

Okay I just noticed I actually put two brackets. I can see now why everyone is confused and guessing. Thanks for correction.
 

FAQ: Arctan limit (without L'Hopital's Rule)

1. What is the definition of the arctan limit?

The arctan limit is a mathematical concept that represents the value that a function approaches as its input approaches a specified value. It is also known as a limit at infinity or a limit at a point.

2. How is the arctan limit calculated without using L'Hopital's Rule?

The arctan limit can be calculated using the following formula: limx->a f(x) = limx->a [tan-1(f(x)) / (x-a)] * This formula can be used to evaluate the limit without using L'Hopital's Rule.

3. What are the common uses of the arctan limit in real life?

The arctan limit has various applications in engineering, physics, and economics. It is used to model and predict the behavior of systems that approach a steady state or equilibrium. It is also used in calculating the rate of change of a function, such as in determining the speed of an object or the growth rate of a population.

4. What are the key properties of the arctan limit?

The arctan limit has several properties, including the fact that it is a one-sided limit, meaning it approaches a value from one direction only. It is also continuous, meaning that the limit at a point is the same as the function value at that point. Additionally, it is monotonic, meaning that it either increases or decreases as the input approaches the specified value.

5. Can the arctan limit be evaluated for any function?

No, the arctan limit can only be evaluated for functions that have a defined arctan function. This includes trigonometric functions, logarithmic functions, and polynomial functions. However, there are some cases where the limit may not exist or may be undefined, such as when the function has a vertical asymptote at the specified value.

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