1) (x+y)³-(x-y)³-2y=x³+3x²y+3xy²+y³ -(x³-3x²y+3xy²-y³)-2y=x³+3x²y+3xy²+y³ -x³+3x²y-3xy²+y³-2y= 6x²y+2y³ -2y=2y(y²+3x²-1)
2) (m+n)³+(m-n)³-2m=m³+3m²n+3mn²+n³+m³-3m²n+3mn²-n³-2m= 2m³+6mn²-2m= 2m(m²+3n²-1)
3) (a-b)³-(c+d)³-a+b+c+d=(a-b-c-d)((a-b)²+(a-b)(c+d)+(c+d)²)-(a-b-c-d)= (a-b-c-d)((a-b)²+(a-b)(c+d)+(c+d)²-1)