$\mathrm{qx}+\mathrm{py}+(\mathrm{p}+\mathrm{q}-\mathrm{r})=0$ is the reflection of the line   $\mathrm{px}+\mathrm{qy}+\mathrm{r}=0$ in the line 

Equation of the bisectors of the angle between the given lines are

$\frac{px+qy+r}{\sqrt{p^2+q^2}}=\pm \frac{qx+py+(p+q-r)}{\sqrt{p^2+q^2}}$

Taking the negative sign we get $x+y+1=0$ 

so one is the reflection of the other line, in the line $x+y+1=0$.