Now, ((a ∗ x) ∗ x) ∗ ((b ∗ (a ∗ x)) ∗ (b ∗ x)) = 1,
by Proposition 3.10. Hence (a ∗ x) ∗ x ≤ (b ∗ (a ∗ x)) ∗ (b ∗ x). Using Lemma 3.6, we have
(b ∗ (a ∗ x)) ∗ (b ∗ x) ∈ I. Since b ∈ I, by (I4), we obtain (b ∗ (a ∗ x)) ∗ x ∈ I. Thus (I2) holds.
Therefore I is an ideal of X.