丁香园AV

In this section we aim to compute the limit:

We start by analyzing the graph of \(\ds{y=\frac{\sin x}{x}}\text{:}\)

We just convinced you this limit formula holds true based on the graph, but how does one attempt to prove this limit more formally? To do this we need to be quite clever, and to employ some indirect reasoning. The indirect reasoning is embodied in a theorem, frequently called the Squeeze Theorem.

This theorem can be proved using the official definition of limit. We won't prove it here, but point out that it is easy to understand and believe graphically. The condition says that \(f(x)\) is trapped between \(g(x)\) below and \(h(x)\) above, and that at \(x=a\text{,}\) both \(g\) and \(h\) approach the same value. This means the situation looks something like 贵颈驳耻谤别听3.2.

For example, imagine the blue curve is \(f(x)=x^2\sin(\pi/x)\text{,}\) the upper (red) and lower (green) curves are \(h(x)=x^2\) and \(g(x)=-x^2\text{.}\) Since the sine function is always between \(-1\) and \(1\text{,}\) \(-x^2\le x^2\sin(\pi/x)\le x^2\text{,}\) and it is easy to see that \(\lim_{x\to0}-x^2=0=\lim_{x\to0}x^2\text{.}\) It is not so easy to see directly (i.e. algebraically) that \(\lim_{x\to0}x^2\sin(\pi/x)=0\text{,}\) because the \(\pi/x\) prevents us from simply plugging in \(x=0\text{.}\) The Squeeze Theorem makes this 鈥渉ard limit鈥� as easy as the trivial limits involving \(x^2\text{.}\)

To compute \(\ds\lim_{x\to0} (\sin x)/x\text{,}\) we will find two simpler functions \(g\) and \(h\) so that \(g(x)\le (\sin x)/x\le h(x)\text{,}\) and so that \(\lim_{x\to0}g(x)=\lim_{x\to0}h(x)\text{.}\) Not too surprisingly, this will require some trigonometry and geometry. Referring to Figure, \(x\) is the measure of the angle in radians. Since the circle has radius 1, the coordinates of point \(A\) are \((\cos x,\sin x)\text{,}\) and the area of the small triangle is \((\cos x\sin x)/2\text{.}\) This triangle is completely contained within the circular wedge-shaped region bordered by two lines and the circle from \((1,0)\) to point \(A\text{.}\) Comparing the areas of the triangle and the wedge we see \((\cos x\sin x)/2 \le x/2\text{,}\) since the area of a circular region with angle \(\theta\) and radius \(r\) is \(\theta r^2/2\text{.}\) With a little algebra this turns into \((\sin x)/x \le 1/\cos x\text{,}\) giving us the \(h\) we seek.

To find \(g\text{,}\) we note that the circular wedge is completely contained inside the larger triangle. The height of the triangle, from \((1,0)\) to point \(B\text{,}\) is \(\tan x\text{,}\) so comparing areas we get \(x/2 \le (\tan x)/2 = \sin x / (2\cos x)\text{.}\) With a little algebra this becomes \(\cos x \le (\sin x)/x\text{.}\) So now we have

Finally, the two limits \(\lim_{x\to0}\cos x\) and \(\lim_{x\to0}1/\cos x\) are easy, because \(\cos(0)=1\text{.}\) By the Squeeze Theorem, \(\lim_{x\to0} (\sin x)/x = 1\) as well.

Using the above, we can compute a similar limit:

This limit is just as hard as \(\sin x/x\text{,}\) but closely related to it, so that we don't have to do a similar calculation; instead we can do a bit of tricky algebra.

To compute the desired limit it is sufficient to compute the limits of the two final fractions, as \(x\) goes to 0. The first of these is the hard limit we've just done, namely 1. The second turns out to be simple, because the denominator presents no problem:

Thus,

Let's do a harder one now.

When solving problems using the Squeeze Theorem it is also helpful to have the following theorem.