sn=a1+a2+a3+……+a(n-1)+an=q+q^2*2+q^3*3+……+q^(n-1)*(n-1)+q^(n+1)*nqsn=q^2+q^3*2+q^4*3+……+q^n*(n-1)+q^(n+1)*n两式相减:(1-q)sn=q+q^2+q^3+q^4+……+q^n-q^(n+1)*n =q[1-q^(n+1)]/(1-q)-q^(n+1)*n=[q-q^(n...