The standard deviation of the following distribution

is

The maximum value of xy when x + 2y = 8 is

If truth values of statements $$\mathrm{p}, \mathrm{q}$$ are true, and $$\mathrm{r}$$, $$s$$ are false, then the truth values of the following statement patterns are respectively

$$\begin{aligned} & \mathrm{a}: \sim(\mathrm{p} \wedge \sim \mathrm{r}) \vee(\sim \mathrm{q} \vee \mathrm{s}) \\ & \mathrm{b}:(\sim \mathrm{q} \wedge \sim \mathrm{r}) \leftrightarrow(\mathrm{p} \vee \mathrm{s}) \\ & \mathrm{c}:(\sim \mathrm{p} \vee \mathrm{q}) \rightarrow(\mathrm{r} \wedge \sim \mathrm{s}) \end{aligned}$$

The rate of change of $$\sqrt{x^2+16}$$ with respect to $$\frac{x}{x-1}$$ at $$x=5$$ is