a(n+1)+x(n+1)+y=2an+n+x(n+1)+y
a(n+1)+x(n+1)+y=2[an+(x+1)/2*n+(x+y)/2]
所以x=1,y=1
a(n+1)+(n+1)+1=2(an+n+1)
所以an+n+1等比,q=2
an=-n-1+(a1+2)*q^(n-1)