(2 points) 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 \[ \frac{1}{R}=\frac{1}{r}+\frac{1}{s} \] where $R, r$, and $s$ are positive. Assume that $s$ is constant. Find $\frac{d R}{d r}$ : \[ \frac{d R}{d r}= \] 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 $a \leq r \leq b$, 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.)

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