Step 1: Construct EF parallel to AB and CD

            NOTE: EF || AB || CD

Step 2:  Apply the basic proportional theorem to triangle ACD and ACB

            THEOREM:  If a line is drawn parallel to one side of a triangle to intersect

            the other two sides in distinct points, then the other two sides are divided

              in the same ratio.

        EXAMPLE:  In ∆ACD, EO || CD

                            \frac{AO}{OC}=\frac{EA}{ED}...................(1)

                        In ∆ACB, EO || AB

                            \frac{DO}{OB}=\frac{ED}{EA}

                            Take the reciprocal on both sides

                        \frac{BO}{DO}=\frac{EA}{ED}  ........................(2)

Step 3: Prove the required ratio by using equation (1) and (2)

            From equation (1) and (2) we can write

                      \frac{AO}{OC}=\frac{BO}{DO}

                      \frac{AO}{BO}=\frac{OC}{OD}

        Hence proved