Big Ideas Math: Modeling Real Life, Grade 7
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5. Finding Unknown Angle Measures
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Exercise 21 Page 395

Angles on the opposite sides of the intersection of two lines are vertical angles.

Classification: Vertical angles.
Value of x: 128

Practice makes perfect
First, let's take a look at the given diagram.

Because the angles in the diagram are on opposite sides of an intersection of two lines, we know that the angles are vertical. The measures of these angles are equal to each other by the Vertical Angles Theorem. x^(∘) = 128^(∘)