(1/3b)-(1/a)=(a-3b)/(3ba)
(a-3b)*(a+3b)/(3ab) : (a-3b)/(3ba)=
= [b]([/b](a-3b)*(a+3b)/(3ab) [b])[/b]* [b]([/b](3ba)/(a-3b) [b])[/b]=
= [b]([/b](a-3b)*(a+3b)*(3ba) [b])[/b] / [b]([/b](3ab)*(a-3b) [b])[/b]=
=a+3b
при a= 3 целых (1/7) b=5 целых 2/7
3 целых 1/7+3*( 5 целых 2/7)= (22/7) + 3*(37/7)=
=(22/7)+(111/7)=133/7= [b]19[/b]