Consider centre of the rectangle as origin and let the diagonals of the rhombus lie along the axis and the axis.The coordinates of the vertices of the rhombus are

The cartesian equations of the sides are respectively

When we substitute in the LHS of the above equations we get i.e. a negative quantity is obtained in all the four cases.Therefore all points inside the rhombus when substituted in the LHS of the above equations must yield a negative quantity.

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