设a=x^2+3x+4,b=2x^2+x+1,则
4(x^2+3x+4)(2x^2+x+1)-(3^2+4x+5)^2=4ab-(a+b)^2=-(a-b)^2=-(x^2-2x-3)^2=-(x-3)^2(x+1)^2