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两等差数列an和bn,前n项和分别为Sn,Tn,且Sn/Tn=7n+2/n+3,则a2+a20/b7+b15=?
题目详情
两等差数列an和bn,前n项和分别为Sn,Tn,且Sn/Tn=7n+2/n+3,则a2+a20/b7+b15=?
▼优质解答
答案和解析
S21=(a2+a20)*21/2
T21=(b7+b15)*21/2
故
a2+a20/b7+b15
=[(a2+a20)*21/2]/[(b7+b15)*21/2]
=S21/T21
=(7*21+2)/(21+2)
=149/23
T21=(b7+b15)*21/2
故
a2+a20/b7+b15
=[(a2+a20)*21/2]/[(b7+b15)*21/2]
=S21/T21
=(7*21+2)/(21+2)
=149/23
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