连续使用L'Hospital Rule
lim x→0 (x－sinx)/(x－tanx)
＝lim x→0 (1－cosx)/(1－(secx)^2)
＝lim x→0 (sinx)/(-2secx·secx·tanx)
＝lim x→0 -(cosx)^3/2
＝-1/2