P.'s as $a_1$ and $a_2$, and their common differences as $d_1$ and $d_2$ respectively. The sum of the first $n$ terms of an A.P. is given by $S_n = \frac{n}{2}[2a + (n-1)d]$. So, we can write the sums of the first $n$ terms of the two A.P.'s as $S_{n1} =
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