n次实系数多项式f(x)的根全为实数,则可以表示成f(x)=a(x-x(1))(x-x(2))...(x-x(n)),a≠0,则f'(x)=a(x-x(2))(x-x(3))...(x-x(n)) + a(x-x(1))(x-x(3))(x-x(4))...(x-x(n)) + ...+ a(x-x(1))...(x-x(n-1)),不妨设x(1) ...