in Lemma 3.2.1, parts 1 to 5 are already in simplest form. I don\'t think you need proof. They are just using commutative property. So you can just add \"commutative property\" as reason for each part Solution in Lemma 3.2.1, parts 1 to 5 are already in simplest form. I don\'t think you need proof. They are just using commutative property. So you can just add \"commutative property\" as reason for each part.