at http://tutorial.math.lamar.edu/Classes/CalcII/ConvergenceOfSeries.aspx in example number 1 how Sn= Summation from i=1 to n of i suddenly becomes n(n+1)/2 how did it happen?

Question

turingtest · Answer

$$\sum_{i=1}^{n}i=\frac{n(n+1)}2$$this is a well known sum that can be proven through many methods
would you like me to do so for you? I like to practice induction.

anonymous · Answer

no thanks

turingtest · Answer

in case you ever get curious http://www.9math.com/book/sum-first-n-natural-numbers and there are other useful ones for the firs n squares and cubes as well http://pirate.shu.edu/~wachsmut/ira/infinity/answers/sm_sq_cb.html