分解因式：x+4x-1
原式=(2x+x^2+1)(2x-x^2-1)=-(x+1)^2(x-1)^2
4x^2-(x^2+1)^2=(2x+x^2+1)(2x-x^2-1)=-(x-1)^2(x+1)^2
4x^2-(x^2+1)^2=4x²-x^4-1-2x²=-x^4-1+2x²=-(x²-1)²=-(x+1)²(x-1)²
