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Triangles and Interior & Exterior Angles

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10 Questions

What type of triangle has three acute angles?

Equilateral triangle

Which type of triangle contains an obtuse angle?

Right triangle

How many sides of equal length does an isosceles triangle have?


Which type of triangle has exactly one obtuse angle?

Isosceles triangle

What are the angles within a triangle called?

Interior angles

What is the sum of the interior angles of any triangle?

$180$ degrees

Why do all triangles have a total measurement of $180$ degrees?

Because a straight angle measures $180$ degrees.

In an equilateral triangle, how much is the measurement of each angle?

$60$ degrees

If one angle in an isosceles triangle is $80$ degrees, what is the measurement of the other two angles?

$40$ degrees each

If angle A in a triangle measures $30$ degrees and angle B measures $60$ degrees, what is the measurement of angle C?

$80$ degrees

Study Notes

Triangle Basics

  • A triangle is a shape with three points and three lines that intersect at these points, containing a total of three interior angles.
  • Interior angles are the angles within the triangle, and exterior angles are the angles that lie outside the triangle.

Types of Triangles

  • Right triangles contain a right angle (90°).
  • Acute triangles have three acute angles (less than 90°).
  • Obtuse triangles have one angle that is greater than 90° (obtuse angle).
  • Equilateral triangles have three equal sides and three equal angles.
  • Isosceles triangles have two sides of the same length and two angles with the same measurement.
  • Scalene triangles have sides that are all unique in size.

Angle Sum Property

  • The sum of the interior angle measurements within a triangle always equals 180°.
  • This is true for any and every triangle, regardless of type.

Why Triangles Have 180 Degrees

  • A straight angle is an angle whose measurement is exactly 180°.
  • If a straight angle is divided into three points, a triangle can be formed, and the sum of the interior angles still equals 180°.

Solving Unknown Angles

  • If one angle measurement is unknown, it can be found by taking the sum of the other two angle measurements and subtracting it from 180°.
  • Using the properties of the triangle type, one can also determine unknown angles.

Learn about the properties of triangles, including interior angles and exterior angles. Understand how interior angles are angles within the triangle, while exterior angles are outside the triangle. Explore examples of interior and exterior angles in triangles.

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