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To find the limits of inverse trig functions, you need to be very familiar with how inverse trig functions work. Remember that \(\sin^{-1}(x) \neq 1/\sin(x)\) in this context. Our expanded site, 17University, has a precalculus section on inverse trig functions.

There are several ways to evaluate limits of inverse trig functions. Of course the most direct way is substitution, which you want to always try first. If that doesn't work, you can move the limit inside the inverse trig function like this.

\(\displaystyle{ \lim_{ x\to c }{ \arcsin(x) } = \arcsin \left[ \lim_{ x\to c }{ x } \right] }\)

This works with the other inverse functions as well since the inverse trig functions are continuous on their restricted domains. (Not sure what we mean? Review this page.)

Another idea is to convert to regular trig functions and try to see what will happen. So, for example, \(\theta = \arcsin(x) \to \sin(\theta) = x\). This will help since you are probably more familar with straight trig functions than you are with inverse trig functions. You may also be able to use trig identities to simplify the equation so that you can then evaluate the limit.

A third idea that will help you see what is going on is to graph the inverse trig function. You don't need to know the graphs off the top of your head since you probably already know the graphs of the trig functions. Just graph the trig functions and flip the graph about the line \(y=x\). In practice problem 2311, this is what he does. Although this technique is probably not enough work to get full credit for finding the limit, it can really help you visualize what is going on and may give you an idea on how to solve the problem.

Okay, try these practice problems.

Practice

Instructions - Unless otherwise instructed, evaluate these limits. Give your answers in exact terms.

\(\displaystyle{ \lim_{x \to -1^+}{\sin^{-1}(x)} }\)

Problem Statement

\(\displaystyle{ \lim_{x \to -1^+}{\sin^{-1}(x)} }\)

Solution

2311 solution video

video by PatrickJMT

close solution

\(\displaystyle{ \lim_{x \to \infty}{\arcsin\left[ \frac{1+x^2}{1+2x^2} \right] } }\)

Problem Statement

\(\displaystyle{ \lim_{x \to \infty}{\arcsin\left[ \frac{1+x^2}{1+2x^2} \right] } }\)

Solution

2312 solution video

video by PatrickJMT

close solution

\(\displaystyle{ \lim_{x \to \infty}{\arctan(e^{3x})} }\)

Problem Statement

\(\displaystyle{ \lim_{x \to \infty}{\arctan(e^{3x})} }\)

Solution

2313 solution video

video by PatrickJMT

close solution

\(\displaystyle{ \lim_{x \to 0^+}{ \arctan(\ln(x))} }\)

Problem Statement

\(\displaystyle{ \lim_{x \to 0^+}{ \arctan(\ln(x))} }\)

Solution

2314 solution video

video by PatrickJMT

close solution
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