a
由题意可得 S13 S7=
13(a1+a13) 2
7(a1+a7) 2= 13(a1+a13) 7(a1+a7) = 13×2a7 7×2a4= 13 7× a7 a4= 13 7×2= 26 7. 故答案为: 26 7
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