\( \definecolor{colordef}{RGB}{249,49,84} \definecolor{colorprop}{RGB}{18,102,241} \)
Let \((u_n)\) be an arithmetic sequence with initial term \(u_0\) and common difference \(d\).
Use the formula for the sum of an arithmetic sequence to prove that:
\(\displaystyle\sum_{i=0}^n u_i = \frac{(n+1)(2u_0 + nd)}{2}\)

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