$\displaystyle b= \frac{a+1}{a \cdot c}$\displaystyle b= \frac{a+1}{a \cdot c}

$\displaystyle \displaystyle b= \frac{a \cdot c}{a+1}$\displaystyle \displaystyle b= \frac{a \cdot c}{a+1}

$\displaystyle b= \frac{a \cdot c}{a-1}$\displaystyle b= \frac{a \cdot c}{a-1}

$\displaystyle \frac{a-1}{a \cdot c}$\displaystyle \frac{a-1}{a \cdot c}