Exercise 13 Page 339

We have been given the measures of the two angles of a triangle, 120 and 30. We will find the measure of the third angle using the Triangle Angle-Sum Theorem. This theorem tells us that the measures of the interior angles of a triangle must add to 180^(∘). Let's add some labels to the given diagram.

We can solve the equation for the measure of ∠ 1.

m∠ 1+ 120+ 30=180

AddTerms

Add terms

m∠ 1+150=180

SubEqn

LHS-150=RHS-150

m∠ 1=30

The measure of the angle numbered with 1 is 30^(∘).