6.
((6a-3ab)/(b²+4b+4)):(3a/(b²-4))=(3a(2-b)/(b+2)²):(3a/((b-2)(b+2))=
=-3a(b-2)(b-2)(b+2)/(3a((b+2)²)=-(b-2)²/(b+2).
Ответ: 3.
7.
(2+√3)/(7+2√12):1/(2+√3)=(2+√3)²/(7+2√3)=(4+4√3+3)/(7+2√12)=
=(7+2√12)/(7+2√12)=1.
8.
((5-x)²/2x)*(6x²/(x²-10x+25))=(x²-10x+25)*6x²/(2x*(x²-10x+25)=3x.