А)
(√15+√5)*√15 - 5/3*√27 = 15+ √75 - 5/3√9*3 = 15 + √25*3 - 5/3*3√3 =
15 + 5√3 - 5√3 = 15,
б)
[(а-2)/(2+а) + (а+2)/(2-а) ] : [ (12а²)/(4-а²) ] =
[ ((а-2)(2-а) + (а+2)(2+а)) / (2+а)(2-а) ] * [(4-a²)/(12a²)] =
[ (2a-a²-4+2a + 2a+a²+4+2a) ] / (12a²) =
8a / 12a² =
(4a*2) / (4a*3a) =
2/3a