Sn=2An - 3n
S(n-1) = 2A(n-1)-3(n-1)
An=Sn-S(n-1)=2An-2A(n-1)+3
An +3 = 2A(n-1)
å°±æ¯ï¼(An-3) = 2[A(n-1)-3]
An-3ææçæ¯æ°åï¼An-3 = (A1-3) *2^(n-1)
An= 3+ (A1-3)*2^(n-1)
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åè¦æ±2^(k-1) + 2^(n-1) = 2^(m-1) *2
æååº2^(k-1)å¾å°
1+2^(n-k) = 2^(m-k+1)
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