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=33、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)y4、∵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=3x-y的算数平方根是√3