Vectors +Tangents # 2

In the rectangular Cartesian system of coordinates $Oxy$ , a tangent is drawn to the curve $y = 2\sqrt x$ at a point A $(x_0,y_0)$ , where $x_0 = 1$ . The tangent cuts the $Ox$ axis at a point B . Find the scalar product of the vectors $\vec{OA}$ and $\vec{AB}$ .