Question 1167120
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Number of terms is the same, as the number of terms in the sequence


    -1, 1, 3, 5, . . . , 19.



Even if you know nothing about arithmetic progressions, and even if you are 2-nd grade student, 

you can solve this problem and answer this question by writing all the terms of the sequence in a row and counting them.



But if you are a bit more advanced thinker, you may figure out that the number of gaps between 1 an 19 is

     {{{(19-1)/2}}} = {{{18/2}}} = 9;



Hence, the number of term in the sequence  1, 3, 5, . . . , 19  is  10;



Then joining the term  {-1},  you get that the number under the problem's question is  10 + 1 = 11.    <U>ANSWER</U>
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