4ab/ab^2-2b=4ab/b(ab-2)=4a/ab-2
при a=√5-1, b=√5+1.
4a/ab-2=4(√5-1)/(√5-1)(√5+1)-2=
=4(√5-1)/5-1-2=4(√5-1)/2=2(√5-1)=2√5-2
