a(n+2)=2a(n+1)-ana(n+2)-a(n+1)=a(n+1)-an所以an为等差数列a4-a1=3dd=-2an=-2n+102.当n《5 ,|an|=-an当n>5 |an|=anSn=n(n+1)+(-1)^sing(n-5)*20或者sn= 当n《5sn=(n-5)(n-4)+20 当n>53.bn=2(1/N-1/(N+1)) 1