The 180° Rule
Slide to see how the angles of a triangle form a straight line.
Exterior Angle Theorem
The exterior angle equals the sum of the two opposite interior angles.
=
+
Find the Missing Angle
Use the Exterior Angle Theorem to calculate the unknown interior angle.
{{ s3_val }}°
Correct!
Parallel Line Properties
Tap the equivalent alternate interior angle inside the triangle.
Shape the Triangle
Drag the top vertex to make the exterior angle exactly .