1.分解因式3a²-27b²

1、分解因式
3a²-27b²
=3(a²-9b²)
=3(a+3b)(a-3b)
2、先化简,再求值：
(a²b-2ab²-b²)/b-(a+b)(a-b)
=[b(a²-2ab-b)]/b-(a+b)(a-b)
=a²-2ab-b-a²+b²
=b²-2ab-b
∴当a=0.5,b=-1时,
原式=1+1+1=3
3、把下列各式分解因式：
(1) ab³-ab
=ab(b²-1)
=ab(b+1)(b-1)
(2)(x²-4x+4)-4(x-2)+4
=(x-2)²-4(x-2)+4
=(x-2)²-4(x-2)+2²
=(x-2-2)²
=(x-4)²
(3)(x+y)(x-2y)-x²+4y²
=(x+y)(x-2y)+(4y²-x²)
=(x+y)(x-2y)+(2y-x)(2y+x)
=(x-2y)(x+y-2y-x)
=(x-2y)y
4、∵x²+y²-2x+4y+5=0
(x²-2x+1)+(y²+4y+4)=0
(x-1)²+(y+2)²=0
且(x-1)²≥0,(y+2)²≥0
∴x-1=0,y+2=0
解得x=1,y=-2
∴x-y=1-(-2)=1+2=3
x-y的算数平方根是√3