Geometry and measuresGCSE / IGCSE

Angles in parallel lines with algebra

Choose the correct angle relationship before solving an equation.

  • Corresponding and alternate angles are equal.
  • Co-interior angles add to .
  • Use the parallel arrows to justify the relationship.

Example 1

The marked lines are parallel. Find .

Alternate angles are equal.Apply this to both sides.
Alternate angles are equal.
Apply this to both sides.

Example 2

The marked lines are parallel. Find .

Co-interior angles add to 180°.Apply this to both sides.Apply this to both sides.
Co-interior angles add to 180°.
Apply this to both sides.Apply this to both sides.

Your turn

Question 1

The marked lines are parallel. Find .

Check answer
Alternate angles are equal.Apply this to both sides.
Alternate angles are equal.
Apply this to both sides.
Question 2

The marked lines are parallel. Find .

Check answer
Co-interior angles add to 180°.Apply this to both sides.Apply this to both sides.
Co-interior angles add to 180°.
Apply this to both sides.Apply this to both sides.
Question 3

The marked lines are parallel. Find .

Check answer
Alternate angles are equal.Apply this to both sides.
Alternate angles are equal.
Apply this to both sides.
Question 4

The marked lines are parallel. Find .

Check answer
Co-interior angles add to 180°.Apply this to both sides.Apply this to both sides.
Co-interior angles add to 180°.
Apply this to both sides.Apply this to both sides.
Question 5

The marked lines are parallel. Find .

Check answer
Alternate angles are equal.Apply this to both sides.
Alternate angles are equal.
Apply this to both sides.
← All Geometry and measures cardsReport a mistake