Question

the measure of an angle is 129°. what is the measure of its supplementary angle?

Explanation:

Step1: Recall the definition of supplementary angles

Supplementary angles add up to 180°. Let the given angle be $\theta = 129^{\circ}$, and the supplementary - angle be $x$. Then $\theta+x = 180^{\circ}$.

Step2: Solve for the supplementary angle

We can find $x$ by subtracting the given angle from 180°. So $x=180^{\circ}-\theta$. Substitute $\theta = 129^{\circ}$ into the formula: $x = 180^{\circ}-129^{\circ}=51^{\circ}$.

Answer:

$51^{\circ}$