I'm having getting errors with my code an I'm not sure how to fix the problem.

(*Define the even function*)

feven[t_] := 1-t

(*Fourier coefficients*)

a[0] = 2/\ Integrate[feven, {t, 0, 1}];

a[n_] := (4/\) Integrate[feven Cos[n \ t], {t, 0, +\/2}];

(*Fourier approximation function*)

fapp[t_, n_] :=

a[0] + Sum[a[2 x - 1] Cos[(2 x - 1) \ t], {x, 1, n}];

(*Plot the function over a time interval *)

Plot[{fapp[t, 6], f}, {t, 0, 1}]

I'm getting error functions:

Integrate::ilim: "Invalid integration variable or limit(s) in {0.0000204286,0,1.}."

NIntegrate::itraw: Raw object 0.000020428571428571424` cannot be used as an iterator.

However when I try things like fapp[1,6] I get accurate and real values. I'm not sure how to fix this problem.