De Morgan's Theorem (2-input):
1) !(A • B) = !A + !B
2) !(A + B) = !A • !B

De Morgan's Theorem (n-input):
1) !(x_1 • x_2 • ... • x_n) = !x_1 + !x_2 + ... + !x_n
2) !(x_1 + x_2 + ... + x_n) = !x_1 • !x_1 • ... • !x_n