When an electric current passes through two resistors with resistance $r$ and $s$, connected in parallel, the combined resistance, $R$, can be calculated from the equation $${1 \over R} = {1 \over {r}} + {1 \over {s}},$$ where $R$, $r$ , and $s$ are positive. Assume that $s$ is constant.

Find $dR\over dr$:
${dR\over dr} =$

Is $R$ and increasing or decreasing function of $r$?
(Enter increasing, decreasing, neither, or both (write both if there are values of $r$ for which $R$ is increasing, and other values for which it is decreasing; enter neither if this is a constant function.)

If we consider the interval $a \le r \le b$, where does $R$ take on its global maximum and minimum values?
maximum: $r =$
minimum: $r =$
(Enter none if there is no global maximum or minimum for this function.)