Question

which equation can be solved to find one of the missing side - lengths in the triangle?
cos(60°)=\frac{a}{12}
cos(60°)=\frac{12}{a}
cos(60°)=\frac{b}{a}
cos(60°)=\frac{12}{b}

Explanation:

Step1: Recall cosine definition

In a right - triangle, $\cos\theta=\frac{\text{adjacent}}{\text{hypotenuse}}$. Given $\angle A = 60^{\circ}$, the hypotenuse is $12$ units and the side adjacent to $\angle A$ is $a$.

Step2: Apply cosine formula

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

Answer:

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