Cosine formula
[tex]\cos (angle)=\frac{\text{ adjacent side}}{\text{ hypotenuse}}[/tex]
Considering angle R, the adjacent side has a length of 9 units, and the hypotenuse of the triangle has a length of 15 units. Substituting this information into the above formula:
[tex]\cos (m\angle R)=\frac{9}{15}=\frac{\frac{9}{3}}{\frac{15}{3}}=\frac{3}{5}[/tex]
