a[n_] := (Integrate[-Cos[n*x], {x, -Pi, 0}] + Integrate[Cos[n*x], {x, 0, Pi}])/Pi;提示SetDelayed::write: "Tag List in {Sin[1],Sin[1/2],Sin[1/3],Sin[1/7]}[n_] is Protected."这是什么意思?