Chapter 1, Equations and Graphs - Section 1.10 - Modeling Variation - 1.10 Exercises - Page 164: 29

$R= \frac{27.5}{\sqrt{x}}$

Since $R$ is inversely proportional to the square root of $x$, we have: $R= \frac{k}{\sqrt{x}}$ We are given: $R=2.5$ and $x=121$, so we have: $2.5= \frac{k}{\sqrt{121}}=\frac{k}{11}$ $k=2.5*11=27.5$ So the equation is: $R= \frac{27.5}{\sqrt{x}}$