令Sn=a1 + 2 *a2 + 2^2 *a3 + 2^3 *a4+……+ 2^n-1* an＝8－5n
所以S（n-1）=a1 + 2 *a2 + 2^2 *a3 + 2^3 *a4+……+ 2^n-2* a（n-1）＝8－5（n-1）=13-5n
所以2^(n-1)*an=Sn-S（n-1）=(8－5n)-(13-5n)=-5
所以an=-5/[2^(n-1)]
a1 + 2 *a2 + 2^2 *a3 + 2^3 *a4+……+ 2^n-1* an＝8－5n 求an
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