Find the missing angle in a triangle

The following diagrams show some triangles with missing interior and exterior angles. Scroll down the page for step by step solutions on how to find the missing angles in triangles.

• Triangle Worksheets

  Printable
  Triangle Sum Theorem
  Exterior Angle Theorem
  Triangle Correspondence
  Explore Triangle Congruence
  Explore SSA Triangle Congruence
  Identify Congruent Triangles
  

  Online
  Missing Angles in a Triangle I
  Missing Angles in a Triangle II
  

Theorems that can be used to find interior and exterior angles of a triangle

1. The Triangle Angle-Sum Theorem:
   The sum of the interior angles of any triangle is always 180°
2. The Exterior Angle Theorem:
   

• An interior angle and its corresponding exterior angle at the same vertex form a linear pair, meaning they are supplementary (sum to 180°).
• The measure of an exterior angle of a triangle is equal to the sum of its two remote interior angles (the two interior angles that are not adjacent to the exterior angle).
• The sum of all exterior angles of a triangle (one at each vertex, extending sides in the same direction) is 360°.

Finding the Missing Angles of a Triangle
How to find the missing interior and exterior angles of a triangle? This video will include solutions to the triangles in the above diagram.

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Angles of Triangles Add Up To 180 Degrees
We can use the property that the angles of a triangle add up to 180 degrees.
Step 1: Find the given angles
Step 2: Subtract the given angles from 180 to get the missing angle.
Ex: Could a triangle have the given angle measures?
a) 115, 35, 30
b) 45, 45, 60

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Angles of Triangles Add Up To 180 Degrees
Since all three angles add up to 180 degrees, you can find the missing angle by subtraction.

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Angles of Triangles Add Up To 180 Degrees
Find two missing angles that are equal.

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How to Find the Missing Angle in a Triangle?

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How to find a missing angle in a triangle?

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