Evaluate the limit of a trigonometric expression

In summary, a limit in trigonometry is the value that a trigonometric expression approaches as the input variable gets closer to a specific value. It can be evaluated using methods such as substitution, graphing, and trigonometric identities. The limit can be undefined if there is a vertical asymptote or if it does not exist due to oscillation or divergence. Some common trigonometric limits include sin(x)/x = 1, tan(x)/x = 1, and cos(x) = 0. These limits can be applied to real-world problems involving angles and periodic functions, such as calculating the velocity of an object in circular motion or determining the position of celestial objects in astronomy.
  • #1
lfdahl
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Evaluate:

\[\lim_{x\rightarrow 0}\frac{\sin (\arctan x)-\tan (\arcsin x)}{\arcsin (\tan x)-\arctan (\sin x)}\]
 
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  • #2
Something like this(?):

Assume \(\displaystyle |x| \approx 0\). Then

\(\displaystyle \begin{align*} \sin (\arctan x)-\tan (\arcsin x) &= \frac{x}{\sqrt{1 + x^2}} - \frac{x}{\sqrt{1-x^2}} \\
&= x\left( 1 - \frac{x^2}{2} + \cdots \right) - x\left( 1 + \frac{x^2}{2} \right) \\
&= -x^3 + \cdots\end{align*}\)

and

\(\displaystyle \begin{align*} \arcsin (\tan x) - \arctan (\sin x) &= \tan x + \frac{\tan^3 x}{6} + \cdots - \sin x + \frac{\sin^3 x}{3} + \cdots \\
&= x + \frac{x^3}{3} + \cdots +\frac{x^3}{6} + \cdots - \left( x - \frac{x^3}{6} + \cdots \right) + \frac{x^3}{3} + \cdots \\
&= x^3 + \cdots\end{align*}\).

Hence

\[\lim_{x\rightarrow 0}\frac{\sin (\arctan x)-\tan (\arcsin x)}{\arcsin (\tan x)-\arctan (\sin x)} = -1.\]
 
  • #3
Theia said:
Something like this(?):

Assume \(\displaystyle |x| \approx 0\). Then

\(\displaystyle \begin{align*} \sin (\arctan x)-\tan (\arcsin x) &= \frac{x}{\sqrt{1 + x^2}} - \frac{x}{\sqrt{1-x^2}} \\
&= x\left( 1 - \frac{x^2}{2} + \cdots \right) - x\left( 1 + \frac{x^2}{2} \right) \\
&= -x^3 + \cdots\end{align*}\)

and

\(\displaystyle \begin{align*} \arcsin (\tan x) - \arctan (\sin x) &= \tan x + \frac{\tan^3 x}{6} + \cdots - \sin x + \frac{\sin^3 x}{3} + \cdots \\
&= x + \frac{x^3}{3} + \cdots +\frac{x^3}{6} + \cdots - \left( x - \frac{x^3}{6} + \cdots \right) + \frac{x^3}{3} + \cdots \\
&= x^3 + \cdots\end{align*}\).

Hence

\[\lim_{x\rightarrow 0}\frac{\sin (\arctan x)-\tan (\arcsin x)}{\arcsin (\tan x)-\arctan (\sin x)} = -1.\]
Hi, Theia! Thankyou for a nice solution and your participation!(Yes)
 

Related to Evaluate the limit of a trigonometric expression

1. What is the definition of a limit in trigonometry?

A limit in trigonometry refers to the value that a trigonometric expression approaches as the input variable gets closer and closer to a specific value. It is denoted as lim x→a f(x), where a is the value that x is approaching and f(x) is the trigonometric expression.

2. How do I evaluate the limit of a trigonometric expression?

To evaluate the limit of a trigonometric expression, you can use a variety of methods such as substitution, graphing, or trigonometric identities. It is important to simplify the expression as much as possible before evaluating the limit.

3. Can the limit of a trigonometric expression be undefined?

Yes, the limit of a trigonometric expression can be undefined if the expression has a vertical asymptote or if the limit does not exist due to oscillation or divergence.

4. What are some common trigonometric limits?

Some common trigonometric limits include lim x→0 sin(x)/x = 1, lim x→0 tan(x)/x = 1, and lim x→π/2 cos(x) = 0. These limits are useful in solving more complex trigonometric limits.

5. How can I use trigonometric limits to solve real-world problems?

Trigonometric limits can be used to model and solve real-world problems involving angles and periodic functions. For example, they can be used in physics to calculate the velocity of an object in circular motion or in astronomy to determine the position of celestial objects.

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