1）a^2+a-56 =（a+8)(a-7)2)2a^2+a-1 =(2a-1)(a+1)3)2x^2+2x-1 =2(x^2+x+1/4-3/4)=2((x+1/2)^2-(√3/2)^2)=2(x+(1/2)+(√3/2))(x+(1/2)-(√3/2))4)x^2-2xy-y^2=x^2-2xy+y^2-(2y^2)=(x+y)^2-(2y^2)=(x+y-√2y)(x+y+√...第四个好像错了吧 真粗心啊！O(∩_∩)O~