${\displaystyle 6dr-2di=0}$

${\displaystyle 3dr=di}$      (2').

${\displaystyle \cos i'di'=n\cos r'dr'}$

${\displaystyle 3\cos i'=ncosr}$      (3').

${\displaystyle 9\cos ^{2}i+\operatorname {sen} ^{2}=n^{2}}$

${\displaystyle 8\cos ^{2}i+1=n^{2}}$

${\displaystyle \cos i={\sqrt {2}}{\frac {n^{2}-1}{8}}}$      (4')

${\displaystyle n={\frac {109}{81}}}$

${\displaystyle i=71^{o}49'55''}$

${\displaystyle r=45^{o}26'50''}$

${\displaystyle D=-50^{o}58'50''}$

${\displaystyle n={\frac {108}{81}}}$

${\displaystyle i=71^{o}26'10''}$

${\displaystyle r=44^{o}47'7''}$

${\displaystyle D=-54^{o}9'38''}$