y'=2xcosx-x²sinxy''=2cosx-4xsinx-x²cosxy^(n) n≥3=x²[cosx]^(n)+2xC(n,1)[cosx]^(n-1)+2C(n,2)[cosx]^(n-2)=x²cos(x+nπ/2)+2nxcos[x+(n-1)π/2]+n(n-1)cos[x+(n-2)π/2]（莱布尼茨公式）...