# Equation or list of equations expected instead of true in the first \ argument

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Problem :

I am facing an error for the BC in this equation:

``a = NDSolve[{D[h[t, x], t] + Sin[x Degree] h[t, x]^2 D[h[t, x], x] + 2/3 Cos[x Degree] h[t, x]^3 == 0, h[0, x] == 1, D[h[t, 0], x] == 0}, h, {t, 0, 100}, {x, 0, 90}]``
``NDSolve::deqn: Equation or list of equations expected instead of True in the first argument {2/3 Cos[[Degree] x] h[t,x]^3+h[t,x]^2 Sin[[Degree] x] (h^(0,1))[t,x]+(h^(1,0))[t,x]==0,h[0,x]==1,True}. >>``

Any tips to resolve it?

## Solution :

I had also faced the similar issue in the recent past. I did lot of research on it and found the solution on it. This is the very common problem with the people getting started.

The last condition as shown below:

`D[h[t, 0], x] == 0`

Is always True as you derivate on the constant. If what you mean is

`D[ h[t,x], x]  /. x->0`

The that's something else.

For the time being, just remove it (and if possible just find another sensible boundary condition compatible with a order of your equation).

`NDSolve[{D[h[t, x], t] + Sin[x] h[t, x]^2 D[h[t, x], x] + 2/3 Cos[x] h[t, x]^3 == 0, h[0, x] == 1}, h, {t, 0, 100}, {x, 0, Pi/2}]`

It works for me.

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