神马作文网已知an=（3n-1)*2^n,求Sn?
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已知an=（3n-1)*2^n,求Sn?
an=（3n-1)*2^n
sn=2*2^1+5*2^2+8*2^3+.+(3n-1)*2^n
2sn=2*2^2+5*2^3+8*2^4+.+(3n-1)*2^(n+1)
sn-2sn=2*2^1+3*2^2+3*2^3+3*2^4+.+3*2^n-(3n-1)*2^(n+1)
-sn=4+12*[1-2^(n-1)]/(1-2)-(3n-1)*2^(n+1)
-sn=4+12*[2^(n-1)-1]-(3n-1)*2^(n+1)
sn=(3n-1)*2^(n+1)-12*[2^(n-1)-1]-4
sn=(3n-1)*2^(n+1)-3*2^(n+1)+8
sn=(3n-4)*2^(n+1)+8
sn=2*2^1+5*2^2+8*2^3+.+(3n-1)*2^n
2sn=2*2^2+5*2^3+8*2^4+.+(3n-1)*2^(n+1)
sn-2sn=2*2^1+3*2^2+3*2^3+3*2^4+.+3*2^n-(3n-1)*2^(n+1)
-sn=4+12*[1-2^(n-1)]/(1-2)-(3n-1)*2^(n+1)
-sn=4+12*[2^(n-1)-1]-(3n-1)*2^(n+1)
sn=(3n-1)*2^(n+1)-12*[2^(n-1)-1]-4
sn=(3n-1)*2^(n+1)-3*2^(n+1)+8
sn=(3n-4)*2^(n+1)+8
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