Big Ideas Math: Modeling Real Life, Grade 8
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2. Angles of Triangles
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Exercise 7 Page 114

The angles inside a triangle are called interior angles.

∠ J = 40^(∘), ∠ Y = 20^(∘)

Practice makes perfect

We can draw a triangle between Jamestown, Williamsburg, and Yorktown. Let's look at the map!

Draw the Historic Triangle

We are given that the interior angle at Williamsburg ∠ W measures 120^(∘) and that the interior angle at Jamestown ∠ J is twice the measure of the angle at Yorktown ∠ Y. We can write the measurements for ∠ J and ∠ Y in terms of the variable x using the given relationship. m∠ W = 120^(∘) , m∠ J= 2x^(∘) , and m∠ Y = x^(∘)

Recall that interior angles of a triangle are the angles inside that triangle. With this information, we can mark the angles ∠ W, ∠ J, and ∠ Y on the diagram.

Next, we can find the measure of the interior angles at Jamestown and Yorktown. 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^(∘).

Using this rule, we can write the equation for the interior angles of the given triangle. 120^(∘) + 2x^(∘)+ x^(∘)=180^(∘) Now, let's solve the equation for x.

120 + 2 x + x = 180
120 + 3 x = 180
3 x = 60
x = 20

We found that x=20. This means that the measure of interior angle at Yorktown is 20^(∘). We can substitute 20 for x to evaluate the expression for the interior angle at Jamestown.

2x^(∘)
2( 20)^(∘)
40^(∘)

The measure of the interior angle at Jamestown is 40^(∘).