Question 1002107
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This arithmetic progression has the common difference d = 77 - 72 = 5 and
{{{a[18]}}} = 77, according to the condition.

{{{a[18]}}} = {{{a[1]}}} + d*(18-1) = {{{a[1]}}} + 5*17 = {{{a[1]}}} + 85.

Thus you have an equation to find {{{a[1]}}}:
{{{a[1]}}} + 85 = 77.

It gives you {{{a[1]}}} = 77 - 85 = -8.
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