How is the “ac” method similar to the “undo FOIL” method? How is it different?

Answer 1

Factoring is the opposite operation from FOILing

there are two method of factoring:

(1) Trial and error, or as you call it "UNDO FOIL"

(2) AC Method

AC Method does not require as much trial and error: you must find two numbers that multiply to AC

but add up to or differ by , depending on the signs, the middle cross term