I do not mean to derail but the more I think about it, the surer I become of the lack of the phrase 「決まってる」 in the active vocabulary of many of the Japanese-learners I have spoken to. Here are some examples of how we use it. And I repeat; We use it ALL THE TIME. Also, remember what particle always precedes this phrase.
Southern Gal: "Y'all have swimming pools in Japan?"
Masaegu: 「あるに決まってんだろっ！
」
Masaegu: "You are the prettiest girl in the world, baby!"
Frisco Gal: "Really? You mean it?"
Masaegu: 「ウソに決まってんだろっ！
」
Tokyo Gal: 「田中君って今週もパーティー行くのかな？」
Masaegu: 「行くに決まってんじゃん。パーティー命だもん、アイ ツ。」
Masaegu: "Did you kiss her?"
Friend: 「したに決まってるだろ。訊くなよ！
」