\(\(\(	\)\(DESARROLLO\ DE\ SERIE\ DE\ FOURIER\ DE\ LA\ FUNCION\ PERIODICA\ f \((t)\) = t\^2, \ CON\  - π < t < π\)\)\)

Clear["Global`*"]

f[t_] := (t^2)

n := 2

s[t_] := ((π^2)/3) + 
        4     (∑_(n = 1)^n)((((-1))^n)/(n^2))*
              Cos[n t]

Plot[{f[t], s[t]}, {t, -π, π}, 
      PlotStyle -> {RGBColor[1, 0, 0], RGBColor[0, 0, 1]}]