Which among following is an example of cyclic amide?

$$\int_\limits0^4 x[x] d x=$$ (where $$[\mathrm{x}]$$ denotes greatest integer function not greater than $$\mathrm{x}]$$

$$\lim _\limits{x \rightarrow 1} \frac{a b^x-a^x b}{x^2-1}=$$

The maximum area of the rectangle that can be inscribed in a circle of radius $$r$$ is