Proposition 2.9. Let x,y,z be any element in a KK-algebra X. Then x∗(y∗ z)=y∗(x∗z). Proof. Since theorem 2.7(2), (x∗z)∗z ≤ x, and by proposition 2.8(1), we get that x∗(y ∗z) ≤ ((x∗z)∗z)∗(y ∗z). Putting x = y and y = x∗z in theorem 2.7(1), it follows that ((x∗z)∗z)∗(y ∗z) ≤ y ∗(x∗z). By the transitivity of ≤ gives x∗(y ∗z) ≤ y ∗(x∗z). And we replacing x by y and y by x, we obtain y ∗(x∗z) ≤ x∗(y ∗z). By the anti-symmetry of ≤, thus x∗(y∗z)=y∗(x∗z) and finishing the proof.