The probability density function of $X$, the lifetime of a certain type of device (measured in months), is given by $$f(x)= \left\{ \begin{array}{ll} 0 & \mbox{if } x\le 9 \cr \frac{9}{x^2} & \mbox{if } x>9 \end{array} \right.$$

Find the following: $P(X>33)$ =

The cumulative distribution function of $X$:

$F(x)=$

$\left.\vphantom{\begin{array}{c}\!\strut\\\!\strut\\\!\strut\\\end{array}}\right\lbrace$ if $x\le 9$ if $x> 9$

The probability that at least one out of 7 devices of this type will function for at least 18 months: