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设数列{an}是等差数列,若a3+a4+a5=12,则a1+a2+…+a7=( ...
设数列{an}是等差数列,若a3+a4+a5=12,则a1+a2+…+a7=( )
A.14
B.21
C.28
D.35
考点分析:
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B.
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