a, x4 + 2x3 + x2 = \(x^2\left(x^2+2x+1\right)=x^2\left(x+1\right)^2=\left[x\left(x+1\right)\right]^2=\)\(\left(x^2+x\right)^2\)
b, x^3 - x + 3x^2y + 3xy^2+y^3-y
x^3 + 3x^2y + 3xy^2+y^3- x - y
(x+y)^3 - (x+y)
=(x+y)[ (x+y)^2 - 1]
=(x+y)(x+y+1)(x+y-1)
c, 5x^2 - 10xy + 5y^2 - 20(c hỗ này có dấu gì ko???) z^2