Segments AB, BC, CD are successively plotted on the straight line. Points E and F are located on opposite sides of this straight line, and the angle ABE is 140 °, the angle ACF is 40 °, the angle FBD is 49 °, the angle ACE is 48 °. that lines BF and CE intersect.
In the quadrangle BECF, the angle CBF = 49, the angle BCE = 48, and since these are cross-lying angles at the intersection of the lines CE and AF of the secant BC, the lines CE and AF are not parallel, and therefore intersect, which was required to be proved.
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