## Geometry: Common Core (15th Edition)

$\cos (A)=\frac{3}{5}$
The cosine of an angle in a right triangle is the ratio of the adjacent side to the hypotenuse. The side adjacent to $A$ has a measure of $6$, and the hypotenuse has a measure of $10$. When we write these in a ratio, we get $\frac{6}{10}$, which simplifies to $\frac{3}{5}$.