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已知数列{an}满足:a1=1,且n为奇数时,an+1=2an,n为偶数时,an+1=an+1,n∈N*.(1)求a2,a3并证明数列{a2n-1+1}为等比数列;(2)求数列{an}的前2n+1项和S2n+1.
题目详情
已知数列{an}满足:a1=1,且n为奇数时,an+1=2an,n为偶数时,an+1=an+1,n∈N*.
(1)求a2,a3并证明数列{a2n-1+1}为等比数列;
(2)求数列{an}的前2n+1项和S2n+1.
(1)求a2,a3并证明数列{a2n-1+1}为等比数列;
(2)求数列{an}的前2n+1项和S2n+1.
▼优质解答
答案和解析
(1)a2=2a1=2,a3=a2+1=3,
∵a2n+1=a2n+1=2a2n-1+1,
∴a2n+1+1=2(a2n-1+1),
∴数列{a2n-1+1}为公比是2的等比数列;
(2)S2n+1=(a1+a2)+(a3+a4)+…+(a2n-1+a2n)+a2n+1+a2n+1
=3a1+3a3+…+3a2n-1+a2n+1
由(1)知,
∴a2n−1+1=2n,
∴a2n−1=2n−1
∴S2n+1=3[(2−1)+(22−1)+…+(2n−1)]+a2n+1=3(2
−n)+2n+1−1=2n+3-3n-7
∵a2n+1=a2n+1=2a2n-1+1,
∴a2n+1+1=2(a2n-1+1),
∴数列{a2n-1+1}为公比是2的等比数列;
(2)S2n+1=(a1+a2)+(a3+a4)+…+(a2n-1+a2n)+a2n+1+a2n+1
=3a1+3a3+…+3a2n-1+a2n+1
由(1)知,
∴a2n−1+1=2n,
∴a2n−1=2n−1
∴S2n+1=3[(2−1)+(22−1)+…+(2n−1)]+a2n+1=3(2
1−2n |
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