4/(√5-1)=4(√5+1)/(√5-1)(√5+1)=4(√5+1)/(5-1)=4(√5+1)/4=√5+1
6/(3+√6)= 6(3-√6)/(3+√6)(3-√6)=6(3-√6)/(9-6)=3-√6
15/(√7+√2)= 15(√7-√2)/(√7+√2)(√7-√2)=15(√7-√2)/(7-2)=15(√7-√2)/5=
=3(√7-√2)
6/(3√2-2√3)= 6(3√2+2√3)/(3√2-2√3)(3√2+2√3)= 6(3√2+2√3)/(18-12)=
=6(3√2+2√3)/6= 3√2+2√3