12. a) 2/√12 + 5/√75 - (3√3/8 + 6/√108 ) =
= 2/2√3 + 5/5√3 - (3√3/8 + 6/6√3) =
= 1/√3 + 1/√3 - ( 3√3/8 + 1/√3)=
= √3/3+√3/3-(3√3/8+√3/3) =
= √3/3+√3/3-17√3/24 =
= -√3/24
b) 6√2/√3 + 3/2√6 + 6√3/3√2 + 4√3/2√2 =
= 6√6/3+3√6/12+2√3/√2+2√3/√2 =
= 2√6 + √6/4 + √2*√3+√3*√2 =
= 2√6 + √6/4 + √6+√6 = 17√6/4
