Comment by mliben

Using the simple drag polar approach,

D = A*v^2 + B/v^2 (D is total drag, first term is parasitic drag, second term is lift-induced drag)

dD/dv = 0 where v = (B/A)^(1/4)

Plug in v = (B/A)^(1/4)

D = sqrt(AB) + sqrt(AB), aka dD/dv = 0 exactly when parasitic drag is equal to the lift-induced drag