Question

1. fill in the blank if the (mangle13 = 60^{circ}) then the measure of (mangle12=?)

Explanation:

Step1: Identify angle - relationship

$\angle12$ and $\angle13$ are supplementary angles (linear - pair of angles).

Step2: Use supplementary - angle property

The sum of two supplementary angles is $180^{\circ}$. Let $m\angle12=x$ and $m\angle13 = 60^{\circ}$. Then $x + 60^{\circ}=180^{\circ}$.

Step3: Solve for $m\angle12$

$x=180^{\circ}-60^{\circ}=120^{\circ}$.

Answer:

$120^{\circ}$