Let P and Q be points on the sides AB and CD respectively
of convex quadrilateral ABCD. Prove the following:

    |AP|     |CQ|
If ------ = ------, then [ABCD] = [QAB] + [PCD]
    |PB|     |QD|

where [KLM...XYZ] denotes the area of polygon KLM...XYZ.