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Counting & ProbabilityEasyIdentities and Techniques, Proved

Identities and Techniques, Proved · Easy problem

The triangular numbers are 1,3,6,10,…1, 3, 6, 10, \ldots, where the nnth one is (n+12)\binom{n+1}{2}. Find the sum of the first ten triangular numbers, 1+3+6+⋯+551+3+6+\cdots+55.