Here $$\mu $$_total = $$\mu $$₁ + $$\mu $$₂ $$ \ne $$ 0

$$\mu $$_total = $$\mu $$₁ $$-$$ $$\mu $$₂ = 0

$$\mu $$_total = $$\mu $$₁ + $$\mu $$₂ $$ \ne $$ 0

$$\mu $$₁ + $$\mu $$₂ + $$\mu $$₃ = $$\mu $$₄

   $$\mu $$_total = 0

In NO₂ and O₃ have permanent dipole moment as both of them are not symmetric molecule.

In CO₂ and SiF₄ each individual bond c = O and S- $$-$$ F have dipole moment but because of symmetric structure individual dipole moment get's cancelled.