Question

12 units
cos(60°) = \frac{12}{a}
cos(60°) = \frac{12}{b}
cos(60°) = \frac{b}{a}
cos(60°) = \frac{a}{12}

Explanation:

Step1: Recall cosine - definition in right - triangle

In a right - triangle, the cosine of an acute angle is defined as the ratio of the adjacent side to the hypotenuse. For angle $B = 60^{\circ}$ in right - triangle $ABC$, the adjacent side to angle $B$ is side $a$ and the hypotenuse is side $AB = 12$ units.

Step2: Write the cosine formula

By the definition of cosine, $\cos(B)=\frac{\text{adjacent}}{\text{hypotenuse}}$. Substituting $B = 60^{\circ}$, the adjacent side $a$ and the hypotenuse $12$, we get $\cos(60^{\circ})=\frac{a}{12}$.

Answer:

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