1.a____________b___________________c______________d________
2.a_________________b________________c___________________________d
3._____a______________b________________c____________________d_______
4.______a______________b_________________c_____________________d
有四种情况,但结果是一样的.
线段有6条:ab,ac,ad,bc,bd,cd.
射线有6条:ab(ac,ad它们三个是同一条射线),bc(bd它们两个是同一条射线),cd,dc(db,da它们三个是同一条射线),cb(ca它们两个是同一条射线),ba.