N=2, N=3についてやってみてなんとなく感じを掴んだら、あとは数学的帰納法で証明する。コツは一度恒等式に代入するところ。

After much research, Donald Gunn, creative director for the advertising agency Leo Burnett, determined that nearly all good ads fall into one of 12 categories -- "master formats," in his words.This slide show presents some recent ads exemp…