27)
a) 1/(a²+ab+b²)+b/(a³ -b³) =1/(a²+ab+b²)+b/(a -b)(a²+ab+b²) = (a-b+b)/(a-b)(a²+ab+b²)
=a/(a³-b³).
в) (x² -2xy)/(x-2y)³ +1/(2y-x) =x(x-2y)/(x-y)³ -1/(x-2y) =x/(x-2y)²-1/(x-2y) =(x -(x-2y))/(x-2y)²
=2y/(x-2y)².
29)
a) 1/(a-b) -1/(b-a) -2a/(a² -b²) = 1/(a-b) +1/(a-b) -2a/(a² -b²) =2/(a-b) -2a/(a-b)(a+b)=
(2(a+b) -2a)/(a-b)(a+b)=2b/((a-b)(a+b).
в) ( (a+b)³+(a-b)³)/2ab(a² +3b²) -1 =(a³ +3a²b+3ab² +b³ +a³ -3a²b+3ab² -b³)/(2ab(a²+3b²)- 1=
(2a³ +6ab²)/2ab(a²+3b²) -1 =(2a³ +6ab²)/2ab(a²+3b²) -1 =2a(a² +3b²)/2ab(a²+3b²) -1 =
1/b -1 =(1-b)/b.
дальше не видно