Question 1167120
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1:  1-k
2:  1+k   (2k was added to term 1)
3:  1+3k  (2k was added to term 2)
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Now looking at this, you can say the n'th term will be (1-k) + 2(n-1)k
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As a check, look at term 3 above, we can plug in n=3:  (1-k) + 2(3-1)k = (1-k) + 4k = 1+3k
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Setting 1+19k = (1-k)+2(n-1)k and re-writing:
        1+19k = 1-3k+2nk   
          19k = -3k+2nk      << subtracted 1 from both sides
          19  = -3+2n        << divided both sides by k
          22  = 2n           << added 3 to both sides
          11  = n            << divided both sides by 2
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n = 11, and n=1 is  the first term, so there are 11 terms in the sequence.