SOLUTION: Use the method of undetermined coef cients to nd the sum of 1 +2+3+···+(n−1)+n,n∈N

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Question 1120395: Use the method of undetermined coef cients to nd the sum of 1 +2+3+···+(n−1)+n,n∈N
Answer by ikleyn(52776) About Me  (Show Source):
You can put this solution on YOUR website!
.
See the lesson
    - Mathematical induction and arithmetic progressions
in this site,  where the formula was proved by the method of Mathematical induction.

See the lesson
    - The proofs of the formulas for arithmetic progressions
in this site,  where the formula was proved in the context of arithmetic progressions.