The error
term for the above Taylor polynomial is, for
ξ=ξ(x) is as in the theorem statement,
R3(x)=4!f(4)(ξ)(x−x0)4=x4(38sin(2ξ)+34ξcos(2ξ))
Evaluated at
x=0.4 this gives
R3(0.4)=1875128sin(2ξ)+64ξcos(2ξ)
for some value of
ξ∈[x0,x]=[0,0.4]. Our goal is to bound the value
R3(0.4); we can do this ala Extreme Value Theorem. The derivative
dξdR3(0.4)=1875320cos(2ξ)−128ξsin(2ξ)
has infinitely many zeros, but within our interval of interest,
[0,0.4], has no zeros. Hence the error
term R3(0.4) is monotonic
with respect to ξ over
[0,0.4], so its extrema lie on the endpoints. We have
R3(0.4)∣ξ=0R3(0.4)∣ξ=0.4=0≈0.0584839
and hence we bound the error of
P3(0.4) as at most
≈0.0584839.
The actual error
∣P3(0.4)−f(0.4)∣ is
≈0.01337.