alternate interior angles

Given: Lines f and g are parallel.

Make a conjecture.

What do you think is the relationship between the angle measures of the highlighted angles?

Test your conjecture.

Click the "Show Angle Measures" button to test your theory from above. Were you correct?

Click and drag point C until lines g and f overlap completely.

What do you notice about the relationship between the two angles of interest?

Tell me more!

Deduce the angle measures of 4 other angles in the diagram.