The symbol to represent energy density is (u).So it is the total energy stored in the capacitor per unit volume.
So according to the above definition its equation will be:
U= Total Energy(U)/ Volume(v)= ½ CV 2/Ad
Using C =A/d and V=Ed.
Now from the above equations we get :
U= A / 2d * E2d /Ad = ½ * E2 ——— 2.18