Exercise 12 Page 339

We have been given the measures of the two angles of a triangle, 59 and 61. 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^(∘).

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

59+ 61+m∠ 1=180

AddTerms

Add terms

120+m∠ 1=180

SubEqn

LHS-120=RHS-120

m∠ 1=60

The measure of the angle numbered with 1 is 60^(∘).