Chapter 13, Trigonometric Ratios and Functions - 13.1 Use Trigonometry with Right Triangles - 13.1 Exercises - Problem Solving - Page 858: 35a

$22817.75$ mile

Consider $r$ to be the radius of the tropic of cancer and that of the earth to be $R$. Then, $\dfrac{r}{R}=\cos \theta$ $R \cos \theta=r$ Since, the circumference of the tropic of cancer is $2 \pi r$. Then, $2 \pi r=2 \pi R \cos \theta $ This gives: $2 \pi (3960) (\cos 23.5)=22817.75$ mile