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Soient \(a,b \in \mathbb{U}\) distincts et \(z \in \C\). On note \(u = \frac{z+ab\overline{z}-a-b}{a-b}\). Montrer que \(u^2 \in \R\).
Soient \(a,b \in \mathbb{U}\) distincts et \(z \in \C\). On note \(u = \frac{z+ab\overline{z}-a-b}{a-b}\). Montrer que \(u^2 \in \R\).