setting a variable equal to the output of FindRoot

Question

So I set a function f[x]

f[x_] := x*E^(-x) - 0.16064

Then I set a variable 'actualroot' to the function FindRoot, starting at 3

actualroot = FindRoot[ f[x], {x, 3} ]

and get the output

{x -> 2.88976}

Later I want to compare this output with a different estimate (-2.88673) of the root, and calculate error, so I have

Abs[ (actualroot - estimateroot)/actualroot ]

and i get this output:

Abs[ (-2.88673 + (x -> 2.88976))/(x -> 2.88976) ]

How do I get mathematica to evaluate this expression? I also tried using N[] to give me a decimal evaluation but it didn't work.

Answer 1

The usual way to get the values of the results of FindRoot, Solve, etc., which are lists of Rule is the following:

f[x_] := x E^(-x) - 0.16064
actualroot = x /. FindRoot[f[x], {x, 3}]
estimateroot = -2.88673;
Abs[(actualroot - estimateroot)/actualroot]

Output:

2.88976
1.99895