Question

c a b 60° b 12 units a cos(60°) = 12/a cos(60°) = 12/b cos(60°) = b/a cos(60°) = a/12

Explanation:

Step1: Recall cosine definition

For an acute angle in a right triangle, $\cos(\theta) = \frac{\text{adjacent side}}{\text{hypotenuse}}$.

Step2: Identify sides for $\angle B$

At $\angle B = 60^\circ$, adjacent side is $a$, hypotenuse is 12 units.

Step3: Substitute into cosine formula

$\cos(60^\circ) = \frac{a}{12}$

Answer:

$\boldsymbol{\cos(60^\circ) = \frac{a}{12}}$ (the fourth option)