(1) If EF⊥BC in F, the rectangle with missing corners is divided into rectangle and trapezoid.

(2) making a trapezoidal center line. The intersection point g between the midpoint h of the rectangle AEFB and the diagonal,

(3) Make a straight line HG, AB and CD intersect at I and J, and take the midpoint K of the straight line IJ.

(4) Make a straight line KO, cross AB in M, and cross CD in N (above need not be written).