Lean-Workbook Syntax 34532: Sum of the first n natural numbers

For every natural $n$, $\sum_{i=0}^{n}i=n(n+1)/2$.

Source: Lean-Workbook row lean_workbook_plus_34532 (Apache-2.0), original record. This corrected declaration replaces the obsolete finite sum/product binder in with ∈ and restores required imports or namespaces. Ranges, casts, quantifiers, and mathematical expressions are preserved.