原式=x⁴-1-4x+4=(x²+1)(x+1)(x-1)-4(x-1)=(x-1)(x³+x²+x+1-4)=(x-1)[(x³-1)+(x²-1)+(x-1)]=(x-1)²(x²+x+1+x+1+1)=(x-1)²(x²+2x+3)