What is the sum of the two interior angles on the same side of the transversal * 1 point?

When two lines intersect they form two pairs of opposite angles, A + C and B + D. Another word for opposite angles are vertical angles.

Vertical angles are always congruent, which means that they are equal.

Adjacent angles are angles that come out of the same vertex. Adjacent angles share a common ray and do not overlap.

The size of the angle xzy in the picture above is the sum of the angles A and B.

Two angles are said to be complementary when the sum of the two angles is 90°.

Two angles are said to be supplementary when the sum of the two angles is 180°.

If we have two parallel lines and have a third line that crosses them as in the ficture below - the crossing line is called a transversal

When a transversal intersects with two parallel lines eight angles are produced.

The eight angles will together form four pairs of corresponding angles. Angles 1 and 5 constitutes one of the pairs. Corresponding angles are congruent. All angles that have the same position with regards to the parallel lines and the transversal are corresponding pairs e.g. 3 + 7, 4 + 8 and 2 + 6.

Angles that are in the area between the parallel lines like angle 2 and 8 above are called interior angles whereas the angles that are on the outside of the two parallel lines like 1 and 6 are called exterior angles.

Angles that are on the opposite sides of the transversal are called alternate angles e.g. 1 + 8.

All angles that are either exterior angles, interior angles, alternate angles or corresponding angles are all congruent.

Example

The picture above shows two parallel lines with a transversal. The angle 6 is 65°. Is there any other angle that also measures 65°?

6 and 8 are vertical angles and are thus congruent which means angle 8 is also 65°.

6 and 2 are corresponding angles and are thus congruent which means angle 2 is 65°.

6 and 4 are alternate exterior angles and thus congruent which means angle 4 is 65°.

Video lesson

Find the measure of all the angles in the figure

Angles on the Same Side of the Transversal

Definition

Angles on the same side of the transversal are angles that are in one of the half-planes formed by the transversal.

Usually this term is combined with interior or exterior angles to define "interior angles on the same side of the transversal" and "exterior angles on the same side of the transversal".

Examples

In each illustration below, LINE 1 is a transversal of LINE 2 and LINE 3. In each illustration below, the following angles are interior angles on the same side of the transversal:

• 
  A and
  
  F
• 
  D and
  
  G

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In each illustration below, LINE 1 is a transversal of LINE 2 and LINE 3. In each illustration below, the following angles are exterior angles on the same side of the transversal:

• 
  B and
  
  E
• 
  C and
  
  H

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Theorem

When a transversal intersects two lines, the two lines are parallel if and only if interior angles on the same side of the transversal and exterior angles on the same side of the transversal are supplementary (sum to 180°).

In illustration one of the example section above, LINE 1 and LINE 2 are parallel and both interior and exterior angles on the same side of the transversal are supplementary.

In illustration two of the example section above, LINE 1 and LINE 2 are NOT parallel and both interior and exterior angles on the same side of the transversal are NOT supplementary.

Demonstration

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