(b/a-a/b)1/b-a=(b^2-a^2)/ab*1/b-a=
=(b-a)(b+a)/(b-a)*ab=(b+a)/ab
a=1/3 b=1/4
(b+a)/ab=(1/4+1/3)/(1/4*1/3)=(3+4)/12 : 1/12=7/12:1/12=7
