Đáp án:
a)VT=x³+y³
³=(x³+3x²y+3xy²+y³)-3x²y-3xy²
=(x+y)³-3xy(x+y)=VP
b)VT=x³-y³
³=(x³-3x²y+3xy²-y³)+3x²y-3xy²
=(x-y)³+3xy(x-y)=VP
c) VT=(x+y)³-(x-y)³
= x³+3x²y+3xy²+y³-x³+3x²y-3xy²+y³
=2y³+6x²y=2y(y²+3x²)
d)x+y+z=0⇒x=-y-z
(x+y+z)³= x³+y³+z³+3(x+y)(y+z)(x+z)
⇒0=x³+y³+z³+3(x+y)(y+z)(x+z)
⇒x³+y³+z³=-3(x+y)(y+z)(x+z)
⇒x³+y³+z³=-3(-y-z+y)(y+z)(-y-z+z)
⇒x³+y³+z³=-3(-z)(-x)(-y)=3xyz
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