（x²＋2x＋3）²
=(x²+2x)²+6(x²+2x)+9
=x²(x+2)²+6x²+12x+9
=x²(x²+4x+4)+6x²+12x+9
=x^4+4x^3+4x²+6x²+12x+9
=x^4+4x^3+10x²+12x+9