Answer: \(S_{56} = 5040\)
\(a + 18d = 52\) ... (I), \(a + 37d = 128\) ... (II)
(II) − (I): \(19d = 76\), \(d = 4\). Then \(a = 52 - 72 = -20\).
\[S_{56} = \frac{56}{2}[2(-20) + 55(4)]\]\[= 28(-40 + 220)\]\[= 28 \times 180 = 5040\]1 answer
Doubtly AI
· 24 Oct 2026
AI-verified · Oct 2026
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