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 10 Page 401

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

80

Practice makes perfect

We are given the following diagram.

The polygon

We are asked to find the value of x. Note 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 quadrilateral is also 360^(∘). Let's mark an exterior angle at each vertex of this polygon.

The polygon with exterior angles marked

The exterior angles of the polygon have measures of x^(∘), 95^(∘), 115^(∘), and y^(∘). These measures sum up to 360^(∘). x^(∘) + 95^(∘) + 115^(∘) + 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 110^(∘) angle form a straight line.

The supplementary angles
Therefore, these angles are supplementary and the sum of their measures is 180^(∘). y^(∘) + 110^(∘) = 180^(∘) Now we can solve this equation for y. For simplicity, we will not write the degree symbol.
y+ 110 = 180
y+ 110-110 = 180-110
y=70
Finally, we will substitue 70 for y into the first equation and solve it for x.
x + 95 + 115 + y = 360
x + 95 + 115 + 70 = 360
Solve for x
x + 280 = 360
x+280-280 = 360-280
x=80
We got that x=80.