Why is it that I can't perform the second plot but the first works just fine? For reference, using Mathematica

Fourier series of t^2 plotted after application of FourierSeries function then % and with the FourierSeries function directly?

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    $\begingroup$ try Plot[Evaluate@FourierSeries[t^2, t, 3], {t, -4, 4}] $\endgroup$ – kglr Nov 22 at 17:11
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    $\begingroup$ Related: mathematica.stackexchange.com/questions/1301/… -- I wonder why FourierSeries[] doesn't give the ivar error. $\endgroup$ – Michael E2 Nov 22 at 20:54
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    $\begingroup$ Execute Attributes[Plot] and you will see that Plot has the attribute HoldAll. The Evaluate inside Plot is needed to override that attribute. $\endgroup$ – Bob Hanlon Nov 22 at 20:58