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
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