a1=1/2a(n+1)+an=2na(n+1) -(n+1) +1/2 = - ( an -n + 1/2 )=> {an -n + 1/2} 是等比数列, q=-1an -n + 1/2 = (-1)^(n-1) .(a1 -1 + 1/2)an = n - 1/2
