Which of the following statements is/are true?
- For any real number $r$ with $|r|>1, \displaystyle{}\lim _{n \rightarrow \infty} \sum_{i=1}^{n} 17 r^{n}=\frac{17}{1-r}$.
- Let $i$ be the positive square root of $-1$ and let $x$ be any real number. Then $$ \tan x=\frac{\left(e^{i x}-e^{-i x}\right)}{i\left(e^{i x}+e^{-i x}\right)} . $$
- $\dfrac{1}{1 \cdot 2}+\dfrac{1}{2 \cdot 3}+\dfrac{1}{3 \cdot 4}+\cdots<1$.
- The function $f(x)$ defined as $$ f(x)=\left\{\begin{array}{cl} 3 \cos 3 x, & 0<x<\pi / 6 \\ 0 & \text { otherwise, } \end{array}\right. $$ is a probability density function.
