lim (1/x^2 - 1/xtanx)
= lim(xtanx - x^2)/(x^3 * tanx) 约去一个tanx
= lim (tanx - x)/x^3 上下都求导数（罗必达法则）
= lim (secx * secx -1)/3x^2
= lim (tanx * tanx)/3x^2
= 1/3