设Sn是等差数列的前n项和,S7=3(a2+a12)则a7/a4的值为?

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设Sn是等差数列的前n项和,S7=3(a2+a12)则a7/a4的值为?
设Sn是等差数列的前n项和,S7=3(a2+a12)则a7/a4的值为?

设Sn是等差数列的前n项和,S7=3(a2+a12)则a7/a4的值为?
S7=7a4
a2+a12=2a7
S7=3(a2+a12)
7a4=3×2a7
7a4=6a7
a7/a4=7/6
提示：本题不用设公差,是考察等差中项性质的,看不懂的话,建议不要马上追问,自己去查一下等差中项性质到底是什么.

S7=a1+a2+....a7=7a4
3(a2+a12)=3*2*a7
7a4=6a7,a7/a4=7/6

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