# Unable to expand derivatives in modelingtoolkit

**URL:** <https://discourse.julialang.org/t/unable-to-expand-derivatives-in-modelingtoolkit/49803>\
**Category:** General Usage\
**Created:** [November 8, 2020, 9:46pm UTC](https://discourse.julialang.org/t/unable-to-expand-derivatives-in-modelingtoolkit/49803 "2020-11-08T21:46:49Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![brett\_knoss](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/brett_knoss/32/13050_2.png) [@brett\_knoss](https://discourse.julialang.org/u/brett_knoss)\
**Post date:** [November 8, 2020, 9:46pm UTC](https://discourse.julialang.org/t/unable-to-expand-derivatives-in-modelingtoolkit/49803/1 "2020-11-08T21:46:49Z")

</div>

```julia
using ModelingToolkit
 @variables x y Δy u μ # these are symbols, they are used in symbolic computing

@derivatives δx'~x δy'~y δu'~u δμ'~μ # these are derivatives, they are assigned with macros

expand_derivatives(δu(sqrt(u))δx(x^2+144)+δμ(sqrt(μ))δy(y^2+144)*Δy)

```

I get an error:

ERROR: MethodError: no method matching (::Differential)(::Num, ::Num)

I think it has something to do with how the brackets are arranged, I though I got it to work and had in save, but I dont’t know what I did different.

expand\_derivatives(δu(sqrt(u))δx(x^2+144)+δμ(sqrt(μ))δy(y^2+144)\*Δy)

---

<div class="post-metadata">

**Author:** ![Mason](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/mason/32/2423_2.png) [@Mason](https://discourse.julialang.org/u/Mason)\
**Post date:** [November 8, 2020, 10:00pm UTC](https://discourse.julialang.org/t/unable-to-expand-derivatives-in-modelingtoolkit/49803/2 "2020-11-08T22:00:53Z")

</div>

The code you posted in your question works fine:

```julia

julia> using ModelingToolkit

julia> @variables x y Δy u μ # these are symbols, they are used in symbolic computing
(x, y, Δy, u, μ)

julia> @derivatives δx'~x δy'~y δu'~u δμ'~μ # these are derivatives, they are assigned with macros
((D'~x), (D'~y), (D'~u), (D'~μ))

julia> expand_derivatives(δu(sqrt(u))δx(x^2+144)+δμ(sqrt(μ))δy(y^2+144)*Δy)
2 * inv(2 * sqrt(u)) * x + 2 * inv(2 * sqrt(μ)) * y * Δy

```

---

<div class="post-metadata">

**Author:** ![brett\_knoss](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/brett_knoss/32/13050_2.png) [@brett\_knoss](https://discourse.julialang.org/u/brett_knoss)\
**Post date:** [November 8, 2020, 10:03pm UTC](https://discourse.julialang.org/t/unable-to-expand-derivatives-in-modelingtoolkit/49803/3 "2020-11-08T22:03:03Z")

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> [@Mason](#):
>
> `expand_derivatives(δu(sqrt(u))δx(x^2+144)+δμ(sqrt(μ))δy(y^2+144)*Δy)`

Ok, it might be my computer then, I got something about Julia crashing, and it’s trying to index Julia, but is stuck loading a C/C++ editor.