The trigonometric functions tangent, sine and cosine are related by the following equation:
[tex]\tan \theta=\frac{\sin \theta}{\cos \theta}[/tex]
In this case, the angle is y°, so:
[tex]\tan y\degree=\frac{\sin y\degree}{\cos y\degree}[/tex]
Since we want the cosine, we can solve for it ans substitute the given tangent and sine:
[tex]\cos y\degree=\frac{\sin y\degree}{\tan y\degree}=\frac{\frac{9}{c}}{\frac{9}{d}}=\frac{9}{c}\cdot\frac{d}{9}=\frac{d}{c}[/tex]
This corresponds to the third alternative.
