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(1)数列{an}中,Sn-2an=2n，①n=1时a1-2a1=2,a1=-2.n>1时S-2a=2(n-1),②①-②，an-2an+2a=2,∴an-2=2(a-2),a1-2=-4,∴{an-2}是等比数列.（2）an-2=-4*2^(n-1)=-2^(n+1),an=b-bn,∴bn-b=a=-2^n,b-b=-2^(n-1),……b2-b1=-2^2,b1=3,累加得bn=3-(2^2+2^3+……+2^n)=3-[2^(n+1)-4]=7-2^(n+1).(3)cn=nbn-2n^2=7... |
