F=AB+CD can be made with 2 ANDs and 1 OR as is obvious. Using DeMorgan's Law, however, we can make the same function using 3 NANDs. Sounds crazy? Yes it does, but here's proof that it works!
The original circuit (top) can be made equivalent to 2 NANDs followed by 2 INVs, then a NOR then 1 INV. Bubble pushing and DeMorgan reduces this to 3 NANDS.
NOTE: Watch out for sneaky parasitic (?) voltage when turning just one input on.
