.（1）X4-1998x2+1999x+1998 修改一下吧
X4-1998x2+1999x-1998
=x^4+x-1998 (x²-x+1)
=x(x+1)(x²-x+1)-1998(x²-x+1)
=(x²-x+1)(x²+x-1998)
（2） x3-3x2-13x+15
=x³-x²-(2x²+13x-15)
=x²(x-1)-(2x+15)(x-1)
=(x-1)(x²-2x-15)
=(x-1)(x+3)(x-5)