The last proof! You need to have proved 07 and 12 before starting this one.

    Now since b and d are not equal to 0 this implies that
    bd not equal to 0 (by 08)
    and 1/(bd) = (1/b)(1/d) (by 12)

       Thus,          ad = bc

              (ad)(bd)-1 = (bc)(bd)-1             

            (ad)(d-1b-1) = (cd)(b-1d-1)           by BR5

            ((ad)d-1)b-1 = c(b(b-1d-1))           by BR6

            (a(dd-1))b-1 = c((bd-1)d-1)           by BR6 

                (a*1)b-1 = c(1*d-1)               by BR5 and BR8

                    ab-1 = cd-1                   by BR5 and BR7

                     a/b = c/d                   notation

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