[(x-y)/(x+2y)]³
=(x-y)³/(x+2y)³
=[(x-y)(x-y)²]/[(x+2y)(x+2y)²]
=[(x-y)(x²-2xy+y²)]/[(x+2y)(x²+4xy+4y²)]
=(x³-2x²y+xy²-x²y+2xy²-y³)/(x³+4x²y+4xy²+2x²y+8xy²+8y³)
=(x³-3x²y+3xy²-y³/(x³+6x²y+12xy²+8y³)