Logical Reasoning - Number Series - Discussion

Very confusing. I've got this solution adding the first 3 numbers (23)and then adding the third, fourth and fifth together
8, 6, 9, 23, 87

8+6+9 = 23.
9+23+87 = 128.

Can anybody explain why this solution is not correct?

Given pro having 8, 6, 9, 23, 87.

We have to do,

8*1 = 8-2 = 6,(8 is the given no, it is in 1 place so 8*1=8 we want next no in the given series we have to subtract 8-2=6).

We follow this method remains all.

No one can predict what comes next, if the series is given like this and the rest of the questions are good but very easy.

One said that by adding the first 3 no's we can get 4th and adding next 3 no's we get 6th but it's wrong. Because if we follow this how we can get 2nd and 3rd no's.

This is good process through which one will improve his/her mental ability.

Is any other easy way except mentioned above all?

25*02+15+54-25*15/25.

It is the best method of develop your knowledge.

General form is X(n+1) = (X (n)*n) - (n+1) ). Where X(n) is the term number in the series, X (n+1) is the subsequent term and n is the element position aka as index.

All algebraic terms will always represent integer values.

X(n); n(X (n)*n)-(n+1)) = X(n+1).

X(1) = 8; n = 1((8)*(1))-(2)) = 6.

X(2) = 6; n = 2((6)*(2))-(3)) = 9.

X(3) = 9; n = 3((9)*(3))-(4)) = 23.

X(4) = 23; n = 4((23)*(4))-(5)) = 87.

X(5) = 87; n = 5((87)*(5))-(6)) = 429.

Therefore.

X(6) = 429; n = 6((429)*(6))-(7)) = 2567.

Any other simpler way?