Standard Limits for Continuity

Rarely Tested

Common limits for continuity at 0:
  • $$ \lim_{x \to 0} \frac{\sin x}{x} = 1 $$
  • $$ \lim_{x \to 0} \frac{1 - \cos x}{x^2} = \frac{1}{2} $$
  • $$ \lim_{x \to 0} \frac{\tan x}{x} = 1 $$
  • $$ \lim_{x \to 0} \frac{e^x - 1}{x} = 1 $$
  • $$ \lim_{x \to 0} \frac{\log(1 + x)}{x} = 1 $$
Question 1

For a real number $$\alpha$$, let $$[\alpha]$$ denote the greatest integer less than or equal to $$\alpha$$. For a finite set $$S$$, let $$|S|$$ denote the number of elements in the set $$S$$.

Consider the functions $$f:(-3,3)\to(-\infty,\,\infty)$$ and $$g:(-3,3)\to(-\infty,\,\infty)$$ defined by

$$f(x)=[x^3]\log_e\big(1+\sin^2(\pi(x-[x])))\big)$$

and

$$g(x)=x^3\sin^2(\pi\log_e(1+x-[x])).$$

Let

$$A=\{x\in(-3,3):f\text{ is discontinuous at }x\}$$

and

$$B=\{x\in(-3,3):g\text{ is discontinuous at }x\}.$$

Then the value of $$|A|+2|B|-|A\cap B|$$ is ___.

Question 2

Let a function $$f: \mathbb{R} \to \mathbb{R}$$ be defined by:
$$f(x) = \begin{cases} \alpha + \beta |x^2 - 3x + 2| & \text{if } x \le 1 \\ \frac{\sin(\pi x)}{x - 1} & \text{if } 1 < x < 2 \\ \gamma x^2 + \delta x + 1 & \text{if } x \ge 2 \end{cases}$$
If $$f(x)$$ is continuous at $$x = 1$$ and at $$x = 2$$, then the value of the expression $$2\alpha  + 4\gamma + 2\delta$$ is equal to:

Question 3

Let $$f:\mathbb R\to\mathbb R$$ be defined by $$f(x)= \begin{cases} \dfrac{1-\cos(x\lfloor x\rfloor)}{x^2}, & x<0 \\[1.2ex] a, & x=0 \\[1.2ex] \dfrac{\ln(1+x)-\sin x+\frac{x^2}{2}}{x^3}, & x>0 \end{cases}$$
where $$\lfloor x\rfloor$$ denotes the greatest integer function.

If $$f(x)$$ is continuous from the right at $$x=0,$$ then which of the following is correct?

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