students estimate the measure of angle ACD and then measure it by using the Angle tool (MENU > Measurement > Angle). Once the Angle tool is selected, the angle is measured by selecting a point on one side of the angle, the vertex point, and then a point on the other side of the angle.

Next, students estimate and measure angle BCE to find that its measure is equals the measure of angle ACD. Have students grab and move line AE to see that this is always true.

Discuss that these angles are called vertical angles and vertical angles are always congruent. Since they are congruent, they have the same measure.    

As an option, you can have students use the Text tool (MENU > Tools > Text) to display the expression a + b and then use the Calculate tool (MENU > Tools > Calculate) to show the sum of two adjacent angles. This will allow students to more readily see that sum of the measures of the linear pairs is, in fact, always 180°.    

Problem two shows two lines, AF and BG, being intersected by another line, CH. Tell students that a line that intersects two or more lines is called a transversal.

The measures of the eight angles formed by the two lines and the transversal are displayed. Point out how all of the pairs of vertical angles have equal measures and all of the linear pairs are supplementary. Be sure students observe that measures of the angles formed by lines AF and CH differ from those formed by lines BG and CH.