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Calculus Level 2

lim x + 1 cos x x = ? \displaystyle \lim_{x \to + \infty} \dfrac{1-\cos x}{x} =?

If you can't directly input the answer into the answer box:

  • input 1234 if you think it approaches + + \infty
  • input 4321 if you think it approaches - \infty
  • input 1324 if the limit does not exist and does not approach either infinity.
  • input 2016 if still none of the above options best describe the answer.


The answer is 0.

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3 solutions

Harsh Khatri
Mar 15, 2016

1 cos x 1 -1\leq \cos x \leq 1

0 1 cos x 2 \displaystyle \Rightarrow 0 \leq 1-\cos x \leq 2

Since the numerator is finite for any real x x we can say that:

lim x + 1 cos x x = k × lim x + 1 x ; k [ 0 , 2 ] \displaystyle \lim_{x\rightarrow +\infty} \frac{1-\cos x}{x} = k \times \displaystyle \lim_{x \rightarrow + \infty} \frac{1}{x} ; k \in [0,2]

lim x + 1 cos x x = k × 0 = 0 \displaystyle \Rightarrow \displaystyle \lim_{x\rightarrow +\infty} \frac{1-\cos x}{x} = k \times 0 = \boxed{0}

Pulkit Gupta
Mar 15, 2016

We rewrite 1 cos x \large 1 - \cos x as 2 sin 2 x 2 \large 2 \sin^2 \frac{x}{2} .

Observe that 2 sin 2 x 2 \large 2 \sin^2 \frac{x}{2} can attain a maximum value of 2 and a minimum of zero. As x approaches infinity, this it keeps on oscillating between its lower and upper bounds.

Thus, the limit would tend to zero as x goes to infinity.

Note that 0 1 cos x x 1 + cos x x 2 x 0\leq\left|\frac{1-\cos x}{x}\right|\leq\frac{1+|\cos x|}{|x|}\leq\frac{2}{|x|}

Now, taking x + x\rightarrow+\infty , we conclude by the squeeze theorem/sandwich theorem that lim x + 1 cos x x = 0 \lim_{x\rightarrow+\infty}\frac{1-\cos x}{x}=\boxed{0}

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