arithmetic series if Sn=50, A1=4, An= -26....Find N.

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for all arithmetic sequences: Sn=n/2(A1+An) and An=A1+ (n-1)d.  (where d=A2-A1=A3-A2=...=An-An-1)   however, if we plug in the numbers, we get N=-50/11 (which is not possible). i dont know which part is wrong, but i can tell you since A1>An and it's an arithmetic series, d should be a minus. I dont think that it is possible to have Sn=50, since N has to be a positive integer--if solved.