F(A,B)=(\overline{A} + \overline{B})(\overline{A + B})=(\overline{A} + \overline{B})\bar{A}\bar{B}=\bar{A}\bar{A}\bar{B}+\bar{B}\bar{A}\bar{B}=\bar{A}\bar{B}+\bar{A}\bar{B}=\bar{A}\bar{B}=\overline{A+B}