A5/B5 ＝（9A5）/（9B5 ）＝（A1+A2+A3+...+A9）/（B1+B2+B3+...+B9 ）＝66/12＝11/2.因为对于等差数列AN BN,A1+A2+A3+...+A9＝9（A1+A9）/2＝4.5（A5+A5）＝9A5,同理B1+B2+B3+...+B9＝9B5
一道不会的数列提AN BN都是 等差数列A1+A2+A3+...+AN / B1+B2+B3+...+BN = 7N+3
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