Question 149
A line $l_1$ intersects the parabola $y = ax^2 + bx + c$ ($a \neq 0$) at the two points $A$ and $B$. A line $l_2$ is parallel to the line $l_1$ and tangent to this parabola at the point $C$. Prove that the arithmetic mean of the abscissas of the points $A$ and $B$ equals the abscissa of the point $C$.