原式=(x^4-2x³+x²)+(5x³-10x²+5x)+(7x²-14x+7)
=x²(x²-2x+1)+5x(x²-2x+1)+7(x²-2x+1)
=(x²-2x+1)(x²+5x+7)
=(x-1)²(x²+5x+7)