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What function could I use to solve this equation for x:

E^(-x^2) = 1 - Cos[x]

I tried Solve, Reduce, Root... None of them worked.

FindInstance gave me an imaginary answer.

Adding Reals, gave me:

{{x -> Root[{-1 + E^-#1^2 + Cos[#1] &, -6.28318531096301494721675292506}]}}

but this is not the answer I wanted, which would be between 0 and 1.

How would I solve this?

As a side note, WolframAlpha got it.

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    $\begingroup$ E.g. Solve[E^(-x^2) == 1 - Cos[x] && 0 <= x <= 1, x] yields {{x -> Root[{1 - E^#1^2 + E^#1^2 Cos[#1] &, 0.94194408148019155746}]}}. See e.g. How do I work with Root objects? $\endgroup$
    – Artes
    Commented Apr 22, 2014 at 22:14
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    $\begingroup$ The reason Solve has a hard time is that it tries to find all solutions, but there are actually infinitely many of them: two each surrounding $x=2n\pi$ for all $n\in\mathbb Z$. (So WolframAlpha's answer is misleading.) Restricting the domain, as in Artes' comment, helps because then there are only a finite number of solutions. $\endgroup$
    – user484
    Commented Apr 22, 2014 at 22:17
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    $\begingroup$ Take a look also at How do I solve this equation? and Can Reduce really not solve for x here?. $\endgroup$
    – Artes
    Commented Apr 22, 2014 at 22:24

1 Answer 1

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This?

NSolve[E^(-x^2) == 1 - Cos[x] && 0 < x < 1, x]
(* {{x -> 0.941944}} *)

Or this?

FindRoot[E^(-x^2) - (1 - Cos[x]), {x, 1}]
(* {x -> 0.941944} *)
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