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anbn=(2^(n+2))*(3n-7)
∴Sn=(-4)*2^3+(-1)*2^4+2*2^5+……+(3n-7)*[2^(n+2)] ①
∴2Sn= (-4)*2^4+(-1)*2^5+2*2^6+……+(3n-7)*[2^(n+3)] ②
②-①得Sn=(-3)*(2^4+2^5+……+2^(n+2))+4*2^3+(3n-7)*[2^(n+3)]
=(3n-7)*[2^(n+3)]+32-3*[2^(n+3)-16]
=(3n-10)*[2^(n+3)]+80
∴Sn=(-4)*2^3+(-1)*2^4+2*2^5+……+(3n-7)*[2^(n+2)] ①
∴2Sn= (-4)*2^4+(-1)*2^5+2*2^6+……+(3n-7)*[2^(n+3)] ②
②-①得Sn=(-3)*(2^4+2^5+……+2^(n+2))+4*2^3+(3n-7)*[2^(n+3)]
=(3n-7)*[2^(n+3)]+32-3*[2^(n+3)-16]
=(3n-10)*[2^(n+3)]+80
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