Question 1197285
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        In this my post I want to explain you  WHY  your problem formulation is  INCORRECT.



When you say  " Find the sum to Infinity of the following Sequence : 18, 12, 8, . . .",

the sequence is,  actually,  UNDEFINED :  we don't know how to continue it.


We only can hypothesize,  what the sequence,  actually,  is,  but making such hypotheses is not the way to solve  Math problems. 



In Math textbooks, such formulations are  NEVER  USED  and are not allowable: 


        NO  ONE  professional  Math writer will write this way.



The correct formulation is &nbsp;" Find the sum to infinity of the following &nbsp;<U>geometric</U> &nbsp;sequence: &nbsp;18, 12, 8, . . .".


Then the sequence is defined by a &nbsp;UNIQUE way, &nbsp;and the problem is good for the solution.



As a joke, &nbsp;I say it every day, &nbsp;that &nbsp;Math composers of this forum got their mathematical education from reading puzzles.



Which means very low level of mathematical education, &nbsp;not sufficient for writing &nbsp;Math problems.