$a) A = x^²(x+y)+y^²(x+y)+2x^²y+2xy^²$
$= (x^²+y^²)(x+y) + 2xy(x+y)$
$= (x+y)(x^²+2xy +y^²)$
$= (x + y)^3$
$b) (x^³+4x^²-x-4) $
$= [(x^³+3x^²+3x+1)+(x^²+4x+4)-8x-8-1] $
$= [(x+1)^³+(x+1)^²-8(x+1)-1] $
$\longrightarrow$ $(x^³+4x^²-x-4) : (x + 1)$
$= (x+1)²+(x+1)-8- $ $\text{$\dfrac{1}{x + 1}$}$