sinx=x-x^3/3!+x^5/5!-...
(tanx)'=(secx)^2,(tan0)'=1
(tanx)"=2(secx)^3(sinx),(tan0)"=0
(tanx)"'=2 cosx (secx)^3+6(sinx)^2(secx)^4,(tan0)"'=2
tanx=x+2x^3/3!+...
sinx-tanx=-x^3/2+...
应该为-x^3/2 呀.