3x^2+6x+3 =3(x²+2x+1)=3(x+1)²-m^2+4mn-4n^2 =-(m²-4mn+4n²)=-(m-2n)²（x+2y)^2-2（x+2y)+1=(x+2y-1)² (x^2 y^2+1）^2-4x^2 y^2=(x²y²+1+2xy)(x²y²+1-2xy)=(xy+...