Question 944285
a[4]=16  and  s[15]=420 
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Equations:
a(1) + 3d = 16
a(1) + 14d = 420
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Subtract and solve for "d"::
11d = 404
d = 404/11
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Solve for a(1)::
a(1) + 3(404/11) = 16
a(1) = 16 - 1212/11
a(1) = 176/11 - 1212/11
a(1) = -1036/11

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Cheers,
Stan H.
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