Answer: The answer is cosine of that acute angle.
Step-by-step explanation: We are to find the ratio of the adjacent side of an acute angle to the hypotenuse.
In the attached figure, we draw a right-angled triangle ABC, where ∠ABC is a right angle, and ∠ACB is an acute angle.
Now, side adjacent to ∠ACB is BC, which is the base with respect to this particular angle, and AC is the hypotenuse.
Now, the ratio is given by
[tex]\dfrac{\textup{adjacent side}}{\textup{hypotenuse}}=\dfrac{BC}{AC}=\dfrac{\textup{base}}{\textup{hypotenuse}}=\cos\textup{ of angle }ACB.[/tex]
Thus, the ratio is cosine of the acute angle.
