1. The problem asks to analyze the sum notation $\sum u_n$ and the expression $S_n = u_2 + u_3 + \cdots + u_{n+2}$.\n\n2. The summation $\sum u_n$ typically means the sum of terms $u_n$ starting from some index, often $n=1$ or $n=0$.\n\n3. The expression $S_n = u_2 + u_3 + \cdots + u_{n+2}$ defines a partial sum starting from the term $u_2$ up to $u_{n+2}$.\n\n4. To relate $S_n$ to a summation notation, we can write:\n$$S_n = \sum_{k=2}^{n+2} u_k$$\n\n5. This means $S_n$ is the sum of terms $u_k$ where $k$ runs from 2 to $n+2$.\n\n6. Therefore, the statement $S_n = u_2 + u_3 + \cdots + u_{n+2}$ is true as it correctly represents the sum of terms from $u_2$ to $u_{n+2}$.\n\nFinal answer: The checkbox option 1. Vraie is correct.