Question 822434
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Hi,
If the 4th term of an arithmetic sequence is 31 and the 8th term of the sequence is 59, find the 1st term of the sequence. 
{{{a[n] = a[1] +(n-1)d}}}
{{{a[4] = a[1] +3d = 31}}}
{{{a[8] = a[1] +7d = 59}}}
 a1 + 7d = 59 
<u> a1 + 3d = 31</u>    |subtracting 2nd EQ from 1st
     4d = 28
      d = 7    and {{{a[1] = highlight_green(10)}}}    (a1 = 59-49)
 CHECKING our answer***
{{{a[4] = 10 + 21 = 31}}}