通项sin(π√(n^2+a^2)) = (-1)^n·sin(π√(n^2+a^2)-πn) = (-1)^n·sin(πa^2/(√(n^2+a^2)+n)).当n > a^2,有0 < πa^2/(√(n^2+a^2)+n) < πa^2/(2n) < π/2.可知此时sin(πa^2/(√(n^2+a^2)+n))恒正,在此范围内...