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# Sum of n terms (advanced)

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## Problem

The sum of the first 6 terms of an arithmetic progression is 138.
The 9, start superscript, start text, t, h, end text, end superscript term and the 14, start superscript, start text, t, h, end text, end superscript term are the ratio 9, colon, 13.
Find the first term, a, start subscript, 1, end subscript.
a, start subscript, 1, end subscript, equals
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