Question 1178529
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Using "a1" to denote the first term is poor form.  Use a(1) or a[1] or something like that.<br>
The form gets really bad when you try to write "ak + 1" for the (k+1)th term.  Use a(k+1) or a[k+1].<br>
The statement (in better form) "a(k+1) = (-2/5)a(k)" means each term is found by multiplying the previous term by (-2/5).  That tells you what the common ratio is.<br>
The n-th term of a geometric sequence is the first term, multiplied by the common ratio (n-1) times: a(n) = a(1)*r^(n-1).<br>
You can do the minimal amount of work to find the answers to your questions.<br>