What if you were told that the polygon is an isoceles trapezoid and that one of its base angles measures ? What can you conclude about its other base angle? After completing this Concept, you'll be able to find the value of a trapezoid's unknown angles and sides.

### Watch This

### Guidance

A
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trapezoid
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is a quadrilateral with exactly one pair of parallel sides.

An
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isosceles trapezoid
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is a trapezoid where the non-parallel sides are congruent.

The base angles of an isosceles trapezoid are congruent. If is an isosceles trapezoid, then and .

The converse is also true. If a trapezoid has congruent base angles, then it is an isosceles trapezoid. The
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diagonals
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of an isosceles trapezoid are also congruent. The
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midsegment (of a trapezoid)
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is a line segment that connects the midpoints of the non-parallel sides:

There is only one midsegment in a trapezoid. It will be parallel to the bases because it is located halfway between them.

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Midsegment Theorem:
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The length of the midsegment of a trapezoid is the average of the lengths of the bases.

If is the midsegment, then .

#### Example A

Look at trapezoid below. What is ?

is an isosceles trapezoid. also.

To find , set up an equation.

Notice that . These angles will always be supplementary because of the Consecutive Interior Angles Theorem.

#### Example B

Is an isosceles trapezoid? How do you know?

, is not an isosceles trapezoid.

#### Example C

Find . All figures are trapezoids with the midsegment marked as indicated.

a)

b)

c)

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Answer:
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a) is the average of 12 and 26.

b) 24 is the average of and 35.

c) 20 is the average of and .

### Guided Practice

an isosceles trapezoid.

Find:

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Answers:
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1. so .

2. is supplementary with , so .

3. By the Triangle Sum Theorem, , so .

4. Since , because they form a linear pair. By the Triangle Sum Theorem, .

### Practice

1. Can the parallel sides of a trapezoid be congruent? Why or why not?

For questions 2-7, find the length of the midsegment or missing side.

Find the value of the missing variable(s).

Find the lengths of the diagonals of the trapezoids below to determine if it is isosceles.