Question: Why does maple rewrite my definition, and does it affect my plot?

Hello

I am trying to plot a harmonic oscillation, but maple keeps rewriting my defintion so when I plot, it gives me a wrong graph...

How do I stop it from doing this, I am assuming it is this that causes the problem, and not the plot command?

Here is what I get:

And here is what I want - made with geogebra.

If it isn't the definition that is the problem, what plot command should I be using then?

With gratitude
Toby

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