qx+py+(p+q−r)=0 is the reflection of the line   px+qy+r=0 in the line

Equation of the bisectors of the angle between the given lines arepx+qy+rp2+q2=±qx+py+(p+q−r)p2+q2Taking the negative sign we get x+y+1=0 so one is the reflection of the other line, in the line x+y+1=0.