Question 388118
What kind of sequence is this? (Arithmetic, geometric, harmonic, ...?)

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From the given (a+2d)(a+7d) = 300.  From the second info, we get {{{(n/2)(2a+ (n-1)d) = 128}}}.  Three variables, two equations.  Hmm. Since you chose not to give the value of n, I think you know the solution to the problem yourself.  So I don't need to continue any further.