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
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