Chapter 6 - Section 6.2 - More on Trigonometric Equations - 6.2 Problem Set - Page 333: 50

$\theta=86.4^o\approx 86^o$

$\;\;\;cos(\theta)=\frac{r^4}{R^4}$ At $\;\;\;\;\;r=2\;\;\;\;\;\;\;\;\;and\;\;\;\;\;\;\;\;\;\;R=4$ $cos(\theta)=\frac{r^4}{R^4}=\frac{16}{256}=\frac{1}{16}$ $cos(\theta)=\frac{1}{16}$ $\theta=cos^{-1}(\frac{1}{16})$ $\theta=86.4^o\approx 86^o$