明明按照例题一个个输入的，不知道为什么会显示错误

y0 = TraditionalForm[y[0]]; y1 = TraditionalForm[y'[0]];
Manipulate[
Plot[Evaluate[
y[t] /. First[
NDSolve[{y''[x] == -x y[x], y[0] == a, y'[0] == b},
y, {x, 0, 4}]]], {t, 0, 4}, PlotRange -> 5], {{a, 1, y0}, -3,
3}, {{b, 0, y1}, -3, 3}]