S1=2a1-2^1 即a1=2a1-2^1,a1=2 S2=2a2-2^2 即a1+a2=2a2-2^2,a2=6 S3=2a3-2^3 即a1+a2+a3=2a3-2^3,a3=16 S4=2a4-2^4 即a1+a2+a3+a4=2a4-2^4,a4=40
设数列{an}的前n项和Sn=2an-2^n,1;求a3,a4
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