GS $P(x)=(x-x_1)(x-x_2)(x-x_3)$$\rightarrow P'(x)=3x^2-2x(x_1+x_2+x_3)+x_1x_2+x_2x_3+x_1x_3$
Ta có: $P'(x_1)=(x_1-x_2)(x_1-x_3)$
TT: $P'(x_2)=(x_2-x_1)(x_2-x_3)$,
$P'(x_3)=(x_3-x_2)(x_3-x_2)$
$\Rightarrow \dfrac{1}{P'(x_1)}+ \dfrac{1}{P'(x_2)}+\dfrac{1}{P'(x_3)}=0$