作业帮这道数列怎么求?第六题
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这道数列怎么求?第六题
这道数列怎么求?第六题
这道数列怎么求?第六题
你是谁
a1=3
bn=b1+(n-1)d
bn=a(n+1)-an
b3=-2, b10=12
To find: a8
solution:
bn=b1+(n-1)d
b3= b1+2d=-2 (1)
b10=b1+9d=12 (2)
(2)-...
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a1=3
bn=b1+(n-1)d
bn=a(n+1)-an
b3=-2, b10=12
To find: a8
solution:
bn=b1+(n-1)d
b3= b1+2d=-2 (1)
b10=b1+9d=12 (2)
(2)-(1)
d=2
from (1), b1=-6
bn= -6+2(n-1)=2n-8
bn=a(n+1)-an
a(n+1) - an =2n-8
an - a(n-1) =2n-10
an - a1 = [ (-6)+(-4)+....+(2n-10) ]
= (n-8)(n-1)
an = n^2-9n+11
a8 = 64-72+11 = 3
Ans: B
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