数学数列问题

证明：因为an+1=2(an-n+1) 所以an+1-2(n+1)=2(an-n) 所以(an+1-2(n+1))/(an-n)=2 所以数列{an-2n}为等比数列 (2)an-2n=(a1-2x1)x2^n-1=2^nbn=2^ncosn*pai, cos1pai+cos2pai=0Tn=T1+T2+T3+……+Tn=2^1xcos1pai+2^2x2pai+2^3x3pai+……+2^(n-1)cos(n-1)*pai+2^ncosn*pai