# Query derivative of DAE solution

**URL:** <https://discourse.julialang.org/t/query-derivative-of-dae-solution/55150>\
**Category:** Modelling & Simulations\
**Tags:** question, diffeq\
**Created:** [February 12, 2021, 3:07pm UTC](https://discourse.julialang.org/t/query-derivative-of-dae-solution/55150 "2021-02-12T15:07:27Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![ohmsweetohm1](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ohmsweetohm1/32/49126_2.png) [@ohmsweetohm1](https://discourse.julialang.org/u/ohmsweetohm1)\
**Post date:** [February 12, 2021, 3:07pm UTC](https://discourse.julialang.org/t/query-derivative-of-dae-solution/55150/1 "2021-02-12T15:07:27Z")

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Is there a way to query the derivative of the solution when solving a DAE?

```julia
function f(out, du, u, p, t)
    out[1] = - 0.04u[1] + 1e4 * u[2] * u[3] - du[1]
    out[2] = + 0.04u[1] - 3e7 * u[2]^2 - 1e4 * u[2] * u[3] - du[2]
    out[3] = u[1] + u[2] + u[3] - 1.0
  end
  u₀ = [1.0, 0, 0]
du₀ = [-0.04, 0.04, 0.0]
tspan = (0.0, 100000.0)

using DifferentialEquations
differential_vars = [true,true,false]
prob = DAEProblem(f, du₀, u₀, tspan, differential_vars=differential_vars)

using Sundials
sol = solve(prob, IDA())

sol.du

```

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<div class="post-metadata">

**Author:** ![ChrisRackauckas](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/chrisrackauckas/32/77_2.png) [@ChrisRackauckas](https://discourse.julialang.org/u/ChrisRackauckas)\
**Post date:** [February 12, 2021, 6:08pm UTC](https://discourse.julialang.org/t/query-derivative-of-dae-solution/55150/2 "2021-02-12T18:08:44Z")

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`sol(t;Val{1})`

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<div class="post-metadata">

**Author:** ![ohmsweetohm1](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ohmsweetohm1/32/49126_2.png) [@ohmsweetohm1](https://discourse.julialang.org/u/ohmsweetohm1)\
**Post date:** [February 17, 2021, 6:58pm UTC](https://discourse.julialang.org/t/query-derivative-of-dae-solution/55150/3 "2021-02-17T18:58:57Z")

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Thanks!  
Is it `sol(t, Val{1})`?

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<div class="post-metadata">

**Author:** ![ChrisRackauckas](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/chrisrackauckas/32/77_2.png) [@ChrisRackauckas](https://discourse.julialang.org/u/ChrisRackauckas)\
**Post date:** [February 17, 2021, 7:08pm UTC](https://discourse.julialang.org/t/query-derivative-of-dae-solution/55150/4 "2021-02-17T19:08:03Z")

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> [@ohmsweetohm1](#):
>
> Is it `sol(t, Val{1})` ?

yes sorry.