高一数学 20 题 会做的写下来拍给我!谢谢!要过程。
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Cn=an+bn
1、cn=an+bn=3/4an-1+1/4bn-1+1+1/4an-1+3/4bn-1=an-1+bn-1+1
cn-1=an-1+bn-1=an-2+bn-2+1
则cn=n-1+a1+b1=n-1+3=n+2
2、an-bn=(an-1-bn-1)/2+1=((an-2-an-2)/2+1)/2+1=……=1+1/2+……+1/(2^(n-2))+(a1-b1)/2=2-1/(2^(n-2))
an=(an+bn+an-bn)/2=(n+2+2-1/(2^(n-2)))/2=(n+4-1/(2^(n-2)))/2
sn=((n+1)n/2+4n-(1/(2^(-1)+1/2^(0)+……1/2^(n-2))))/2=n(n+9)/4+(4-1/(2^(n-2)))/2=n(n+9)/4+2-1/(2^(n-3))
1、cn=an+bn=3/4an-1+1/4bn-1+1+1/4an-1+3/4bn-1=an-1+bn-1+1
cn-1=an-1+bn-1=an-2+bn-2+1
则cn=n-1+a1+b1=n-1+3=n+2
2、an-bn=(an-1-bn-1)/2+1=((an-2-an-2)/2+1)/2+1=……=1+1/2+……+1/(2^(n-2))+(a1-b1)/2=2-1/(2^(n-2))
an=(an+bn+an-bn)/2=(n+2+2-1/(2^(n-2)))/2=(n+4-1/(2^(n-2)))/2
sn=((n+1)n/2+4n-(1/(2^(-1)+1/2^(0)+……1/2^(n-2))))/2=n(n+9)/4+(4-1/(2^(n-2)))/2=n(n+9)/4+2-1/(2^(n-3))
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