当A1A2≠0时,k1= -B1/A1,k2= -B2/A2,
因为A1A2+B1B2=0,所以k1·k2=(B1B2)/(A1A2)= -1,从而L1⊥L2.
若两直线之一与y轴平行,设L1‖y轴,则k1不存在,A1=0,B1≠0.
故由A1A2+B1B2=0得B2=0,k2=0,L2‖x轴,亦有L1⊥L2.
综上,L1⊥L2.