Angles on one side of a straight line

Wednesday, September 16, 2020


Angles on one side of a straight line

The angles on one side of a straight line always add to 180 degrees.

When a line is cut into two and we know one angle, we can always find the other one.

The two angles are supplementary, which means they add to 180 degrees.

For example, if one angle is 40 degrees we know the other angle is 140 degree.

Angle a equals 180°- 40° = 140°


or if one angle is 80 degrees we know the other angle will be 100 degrees.


Angle a equals 180° - 80° = 100°

This method can be used for several angles on one side of a straight line.

Angles on a straight line


Example: What is angle "b" in the picture above?

Angle b is 180° minus the sum of the other angles.

Sum of known angles = 40° + 20° + 15° = 75°

Angle b = 180° − 75°

Angle b = 105°

In summary the angles on one side of a line 180 degrees.

Related Links

Alternate Exterior Angles

Types of Triangles









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