Đáp án:
Giải thích các bước giải:
a) x³+1/27=(x+1/3)(x²-1/3x+1/9)
CT: a³+b³=(a+b)(a²-ab+b²)
b)(a+b)³-(a-b)³
(a+b)³-(a-b)³=(a+b-a+b)((a+b)²+(a+b).(a-b)+(a-b)²)
=2b(a+b).(a-b)+(a-b)²)
CT: a³-b³=(a-b)(a²+ab+b²)
c) (a+b)³+(a-b)³
=(a+b+a-b)((a+b)²-(a+b)(a-b)+(a-b)²)
CT: a³+b³=(a+b)(a²-ab+b²)
d) 8x³+12x²y+6xy²+y³
=(2x)³+4.3.x²y+2.3xy²+y³
=(2x+y)³
CT: (a+b)³=a³+3a²b+3ab²+b³
c)-x³+9x²-27x+27
=-(x³-9x²+27x-27)
=-(x³-3.3.x²+3.9x-3³)
=-(x-3)³
CT: (a-b)³=a³-3a²b+3ab²-b³
