Solve. For Exercises 83 and $84,$ see the second Concept Check in this section, for Exercises 85 and $86,$ see the third Concept Check.
In our definition of division for
$$
\frac{P}{Q} \div \frac{R}{S}
$$
we stated that $Q \neq 0, S \neq 0,$ and $R \neq 0 .$ Explain why $R$ cannot equal 0.