# How to predict X given a set of Ys in Julia?

**URL:** <https://discourse.julialang.org/t/how-to-predict-x-given-a-set-of-ys-in-julia/64746>\
**Category:** New to Julia\
**Tags:** interpolations\
**Created:** [July 16, 2021, 12:41pm UTC](https://discourse.julialang.org/t/how-to-predict-x-given-a-set-of-ys-in-julia/64746 "2021-07-16T12:41:51Z")\
**Posts on this page:** 2\
**Page:** 2

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**Author:** ![gustaphe](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/gustaphe/32/18174_2.png) [@gustaphe](https://discourse.julialang.org/u/gustaphe)\
**Post date:** [July 28, 2021, 1:22pm UTC](https://discourse.julialang.org/t/how-to-predict-x-given-a-set-of-ys-in-julia/64746/21 "2021-07-28T13:22:30Z")

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That’s not the inverse, it’s the derivative. Guessing the calculator just didn’t understand what you meant/couldn’t perform it.

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**Author:** ![Luigi\_Marongiu](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/luigi_marongiu/32/7909_2.png) [@Luigi\_Marongiu](https://discourse.julialang.org/u/Luigi_Marongiu)\
**Post date:** [July 28, 2021, 1:31pm UTC](https://discourse.julialang.org/t/how-to-predict-x-given-a-set-of-ys-in-julia/64746/22 "2021-07-28T13:31:51Z")

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And you are very right! Now I get:

```julia
julia> fit_x
11.990348572842814

```

with the correct interpolation:

 ![CASE_1](https://global.discourse-cdn.com/julialang/original/3X/7/f/7f87b21a21e35404f3424aa4f7ce115f0f9d3828.png)  
Thank you!

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