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A phone is on sale for \(400\) euros in 2019. Each year, its price decreases by \(10\pourcent\) compared to the previous year. Let \(u_n\) be the price of the phone in the year \(2019 + n\).
  1. Give the values of \(u_0\) and \(u_1\).
  2. Express \(u_{n+1}\) in terms of \(u_n\). Deduce the nature of the sequence \((u_n)\).
  3. Deduce the expression of \(u_n\) in terms of \(n\).

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