When two lines intersect, * * * has (2) antipodal angles; Three straight lines intersect at one point, and * * * has (3) antipodal angles; Four straight lines intersect at one point, and * * * has (4) antipodal angles; N straight lines intersect at one point, and * * * has (n) antipodal angles.

It is easy to know that two straight lines intersect. Later, one more straight line intersects at one point, that is, a pair of diagonal lines is divided into two pairs, that is, every other straight line has one more diagonal line.