Big Ideas Math: Modeling Real Life, Grade 8
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2. Angles of Triangles
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Exercise 21 Page 116

The measure of an exterior angle of a triangle is equal to the sum of the measures of the two nonadjacent interior angles.

126^(∘)

Practice makes perfect

We are given a triangle and an exterior angle formed by the support for a window air-conditioning unit. We are asked to find the measure of the exterior angle.

First, let's identify the exterior angle on the given diagram.

triangle
To find the measure of the exterior angle, we will use the rule for the exterior angle measures of a triangle.

Exterior Angle Measures of a Triangle

The measure of an exterior angle of a triangle is equal to the sum of the measures of the two nonadjacent interior angles.

We will now identify the two nonadjacent interior angles.

triangle

The nonadjacent interior angles are a right angle and 3x^(∘). We can write an equation to relate the exterior angle measure to the interior angles. Recall that the measure of a right angle is 90^(∘). exterior angle measure= 3x^(∘)+ 90^(∘) We need find the value of x to find this sum. We will do this using the rule for the interior angle measures of a triangle.

Interior Angle Measures of a Triangle

The sum of the interior angles of a triangle is 180^(∘).

By this rule, we can write the equation for the interior angles of the given triangle. (5x-6)^(∘) + 3x^(∘)+ 90^(∘)=180^(∘) Now, let's solve the equation for x.
(5x-6)+3x+90=180
5x-6+3x+90=180
5x-6+3x=90
Solve for x
8x-6=90
8x=96
8x/8=96/8
x=12
We found that x= 12. We can substitute 12 for x to evaluate the expression for the exterior angle. 3x^(∘)+ 90^(∘) ⇒ 3( 12)^(∘)+90^(∘) Let's simplify this expression!
3( 12)^(∘)+90^(∘)
36^(∘)+90^(∘)
126^(∘)
The measure of the exterior angle is 126^(∘).