摘要：下列由左到右的变形.属于因式分解的是 A.a(x+y)=ax+ay B.ax2-9a=a(x+3)(x-3) C.x2-y2-1=(x-y)(x+y)-1 D.x2+2xy+y2-1=x(x+2y)+(y+1)(y-1) 答案:B

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