(x的二次方+4x+3）（x的二次方+12x+35）+15
=(x+1)(x+3)(x+5)(x+7)+15
=[(x+1)(x+7)][(x+3)(x+5)]+15
=(x²+8x+7)(x²+8x+15)+15
=(x²+8x)²+22(x²+8x)+105+15
=(x²+8x)²+22(x²+8x)+120
=(x²+8x+12)(x²+8x+10)
=(x+2)(x+6)(x²+8x+10)