Isosceles Triangle Theorem Worksheet
Explore and apply the Isosceles Triangle Theorem to find unknown angles and side lengths in triangles.
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Isosceles Triangle Theorem
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Read each question carefully and answer to the best of your ability. Show all your work for full credit.
An isosceles triangle is a triangle that has at least two sides of equal length. The angles opposite these equal sides are also equal. These are called the base angles.
1. An isosceles triangle has at least two sides of length.
2. The angles opposite the equal sides in an isosceles triangle are also .
3. These equal angles are often referred to as the angles.
4. If two sides of a triangle are equal, then the angles opposite those sides are:
Complementary
Supplementary
Equal
Acute
5. In an isosceles triangle, if one base angle measures 50 degrees, what is the measure of the other base angle?
6. Find the value of x in the isosceles triangle below:
7. All equilateral triangles are also isosceles triangles.
True
False
8. A triangle with angles measuring 30°, 30°, and 120° is an isosceles triangle.
True
False
9. An isosceles triangle has a vertex angle of 80 degrees. What are the measures of its base angles?
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