Angle Relationships

Adjacent angles share a vertex and a side, but no point.

Vertical angles are formed by two intersecting lines and are opposite each other. They always have the same measure.

Corresponding angles lie on the same side of the transversal in corresponding positions.

Alternate interior angles lie on the interior of a pair of lines and on opposite sides of the transversal.

When the lines are parallel, corresponding angles are equal. Pairs of alternate interior angles are also congruent.

This is useful when we need to find out the measure of an angle. For instance: Let Angle 1 = (x + 80), Angle 2 = (x + 20)

We should be able to see that as adjacent angles, 1 & 2 form a straight line. Straight lines = 180 degrees. (This also makes the two angles supplementary).

Once we have determined this, we can set up an equation:

(x + 80) + (x + 20 )= 180 Combine like terms.

2x + 100 = 180 Isolate the variable

2x + 100 - 100 = 180 - 100

2x = 80

x = 40 Plug in the value of x and check your work.

40 + 80 + 40 + 20 = 180 Correct!

Try worksheet 9-2 tonight. We'll review it tomorrow.