If the eqations of two intersecting straight lines are L1=a1x+b1y+c1=0 &L2=a2x+b2y+c2=0,then eqations of bisectors of angles between L1=0 & L2=0 are

(a1x+b1y+c1)/sqrt(a12 +b12)=+or-(a2x+b2y+c2=0)/sqrt(a22+b22)

(I had a problem of understanding proof of this teorem)