数列填空2,-8.,0,64.,()A64 B128 C156 D 250
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![数列填空2,-8.,0,64.,()A64 B128 C156 D 250](/uploads/image/z/1269947-11-7.jpg?t=%E6%95%B0%E5%88%97%E5%A1%AB%E7%A9%BA2%2C-8.%2C0%2C64.%2C%EF%BC%88%EF%BC%89A64+B128+C156+D+250)
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数列填空2,-8.,0,64.,()A64 B128 C156 D 250
数列填空
2,-8.,0,64.,()
A64 B128 C156 D 250
数列填空2,-8.,0,64.,()A64 B128 C156 D 250
128
-2,-8,0,64(?)根据数字找规律
-2=[(1-3)/2]*2^1
-8=[(2-3)/2]*2^4
0=[(3-3)/2]*2^x
64=[(4-3)/2]*2^7
=[(5-3)/2]*2^y
找2的指数1,4,x,7,y的规律
设通项b(n)=Ln^2+Mn+N,有
b(1)=1=L+M+n
b(2)=4=4L+2M+n
b(4)=7=16L+4M+n
解得
L=-1/2,M=9/2,N=-3
b(n)=-n^2/2+9n/2-3
n=3时,b(3)=...=6
n=5时,b(5)=...=7
x=b(3)=5
y=b(5)=7
=[(5-3)/2]*2^y=2^7=128