Observe the difference and look for the right vertex angle, When there are two lines intersecting at a point, there are () pairs of vertex angles. When there are three lines, there are () pairs of vertex angles. When there are four lines, there are () pairs of vertex angles. When there are n lines, there are () pairs of vertex angles
When there are two lines intersecting at a point, there are (2) pairs of vertex angles. When there are three lines, there are (6) pairs of vertex angles. When there are four lines, there are (12) pairs of vertex angles. When there are n lines, there are n (n-1) pairs of vertex angles