), and by interchanging
and
in the above formulae we find
Hence by the second part of the principle
[51. Transitivity of Congruence]. [51.1] It follows, from (iii) of [49.7], and from (ii) and (iii) of [50.1], and from (i), (ii), (iii) of [50.3], that equations (i) of [50.1] can be written
