Cumulative Standards Review
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Use the Triangle Exterior Angle Theorem.
m∠1=42
We want to find the missing angle of the given triangle. In order to do so, we will use the Triangle Exterior Angle Theorem. This theorem tells us that the measure of an exterior angle is equal to the sum of its two remote interior angles.
m∠ 1=m∠ 2+m∠ 3 Now, let's apply this theorem to the given triangle.
The sum of the measures of the two remote angles, m∠ 1 and 78^(∘), will give us 120^(∘). 120=m∠ 1+78 ⇒ m∠1=42