已知数列{an}满足a1=1,an+1={2an n为奇数,an+2,n为偶数},且a1+a3+a5+.+a2k--=3
已知数列{an}满足a1=1,an+1={2an n为奇数,an+2,n为偶数},且a1+a3+a5+.+a2k--=3049,则正整数k的值为
赞 d8032 幼苗
共回答了21个问题采纳率：85.7% 举报
a(n+1)=an+2 ; n is even
= 2a(n-1) + 2
a(n+1) +2= 2(a(n-1) +2)
[a(n+1) +2]/(a(n-1) +2) =2
[a(n+1) +2] / (a1+2) = 2^(n/2)
a(n+1) +2 = 3.2^(n/2)
a(n+1) = -2+3.2^(n/2) ; n is even
n is even => n+1 is odd
an = -2+ 3.2^[(n-1)/2] ; n is odd
a1+a3+a5+.+a(2k-1)=3049
(-2+3.2^(0))+ (-2+3.2^(1)) +(-2+3.2^(2))+.+(-2+3.2^(k-1)) =3049
-2k + 3(2^k-1) =3049
k=10
= 2a(n-1) + 2
a(n+1) +2= 2(a(n-1) +2)
[a(n+1) +2]/(a(n-1) +2) =2
[a(n+1) +2] / (a1+2) = 2^(n/2)
a(n+1) +2 = 3.2^(n/2)
a(n+1) = -2+3.2^(n/2) ; n is even
n is even => n+1 is odd
an = -2+ 3.2^[(n-1)/2] ; n is odd
a1+a3+a5+.+a(2k-1)=3049
(-2+3.2^(0))+ (-2+3.2^(1)) +(-2+3.2^(2))+.+(-2+3.2^(k-1)) =3049
-2k + 3(2^k-1) =3049
k=10
1年前
4
可能相似的问题
-
已知数列{AN}满足A1=1,AN+1=2AN+2的N次方.
1年前2个回答
你能帮帮他们吗