f(x)=x4−x3+x2−x+1
f(0)=1
f′(x)=4x3−3x2+2x−1
f′′(x)=12x2−6x+2=2(6x2−3x+1)
We can see that the second derivative doesn't go to zero for any x.
Thus, first derivative is an monotonically increasing function.
First derivative is zero at 0.6<x<0.7
The function gives f(0.6)>0 and f(0.7)>0
So, f(x)≠0 for all x