2) = 2( x² + 2x + y² +2) = 2[(x+1)²+y²]
3) = (x+y)²-4² = (x+y+4)(x+y-4)
4) = x²+x-6 = x²+3x-2x-6 = x(x+3)-2(x+3) = (x-2)(x+3)
5) Cách 1: x²+5x-6 = x²+6x-x-6 = x(x+6)-(x+6) = (x-1)(x+6)
Cách 2: áp dụng Vi-ét a+b+c ta có $\left \{ {{x=1} \atop {x=-6}} \right.$ ⇒ (x-1)(x+6)
6) = x(x²-1) = (x-1)x(x+1)
7) = x(x²-4) = (x-2)x(x+2)
8) = (x-y-1)(x+y-1)
9) = (x-2-3)(x-2+3) = (x-5)(x+1)
10) = (x+3y)(x-3y)
11) = (2x+1-2)(2x+1+2) = (2x-1)(2x+3)
