Glencoe Math: Course 3, Volume 2
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Glencoe Math: Course 3, Volume 2 View details
4. Polygons and Angles
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Exercise 11 Page 401

In a polygon, the sum of the measures of the exterior angles — one at each vertex — is 360^(∘).

130

Practice makes perfect

We are given the following diagram.

The polygon

We are asked to find the value of x. Notice that the x^(∘) angle is one of the exterior angles of the polygon in the diagram. To find the measure of the x^(∘) angle, we will first recall an important piece of information.

Exterior Angles of a Polygon

In a polygon, the sum of the measures of the exterior angles — one at each vertex — is 360^(∘).

This means that the sum of the exterior angles of the given triangle is also 360^(∘). Let's mark an exterior angle at each vertex of this triangle.

The polygon with exterior angles marked

The exterior angles of the triangle have measures of x^(∘), 150^(∘), and y^(∘). These measures add up to 360^(∘). x^(∘) + 150^(∘) + y^(∘) = 360^(∘) We can use this equation to find the value of x, but we need to find the value of y first. Notice that the y^(∘) angle and the 100^(∘) angle form a straight line.

The supplementary angles
Therefore, these angles are supplementary and the sum of their measures is 180^(∘). y^(∘) + 100^(∘) = 180^(∘) Now we can solve this equation for y. For simplicity, we will not write the degree symbol.
y+ 100 = 180
y+ 100-100 = 180-100
y=80
Finally, we will substitue 80 for y into the first equation and solve it for x.
x +150 + y = 360
x + 150+ 80 = 360
Solve for x
x + 230 = 360
x+230-230 = 360-230
x=130
We got that x=130.