# Problem with Manipulate and FourierTrigSeries

I'm facing a problem with Manipulate here, It just plots a blank xy-axis.

Manipulate[
Plot[FourierTrigSeries[-x^2 + a x, x, 10], {x, -2 Pi, 2 Pi}],
{a, -10, 10}]

• You need to add Evaluate[]: Plot[Evaluate[FourierTrigSeries[(* stuff *)], (* stuff *)] – J. M. is away Oct 1 '18 at 19:18
• i didn't understand, what does evaluate do in this case? – Hugo David Costa Pereira Oct 1 '18 at 19:23

J.M.'s advice, given in his comment, is one solution. Here is another using the plot option Evaluated. I also add custom x-axis ticks and give the option Appearance -> "Labeled" to the control variable a to make the manipulation more user friendly.

Manipulate[
Plot[FourierTrigSeries[-x^2 + a x, x, 10], {x, -2 Pi, 2 Pi},
Ticks -> {Subdivide[-2 Pi, 2 Pi, 8], Automatic},
Evaluated -> True],
{a, -10, 10, .2, Appearance -> "Labeled"}]