1.(x+y)2-6(x+y)+9 =(x+y)2-2(x+y).3+32=(x+y-3)2
2. câu này phải là 4x2-36x+812,nếu vậy thì
4x2-36x+812=(2x)2-2.2x.9+92=(2x-9)2
3.9x2-24xy+15y2=(3x)2-2.3x.4y+(4y)2-y2
=(3x-4y)2-y2=(3x-4y-y)(3x-4y+y)=(3x-5y)(3x-3y)
4.a2-25b2+50bc-25c2
=a2-(25b2-50bc+25c2)=a2-[(5b)2-2.5b.5c+(5c)2]
=a2-(5b-5c)2=(a-5b+5c)(a+5b+5c)