1+x-x^3-x^4
=(1+x)-x^3(1+x)
=(1+x)(1-x^3)
＜0
(x+1)(x^3-1)＞0
(x+1)(x^3-1)=(x+1)(x-1)[(x+1/2)^2+3/4]＞0
(x+1)(x-1)＞0
(+)(+)=(+),(-)(-)=(+)
x＞1或x＜-1