[f(x)/g(x)]'=[f'(x)g(x)-f(x)g'(x)]/g^2(x)
分子求导：
[(cosx-xsinx)]'=-sinx-[sinx+xcosx]=-2sinx-xcosx
分母求导：
[(sinx+xcosx)]'=cosx+[cosx-xsinx]=2 cosx-xsinx
[(cosx-xsinx)/(sinx+xcosx)]'
=[(-2sinx-xcosx)*(sinx+xcosx)-(cosx-xsinx)*(2cosx-xsinx)]/(sinx+xcosx)^2
=(-2-x^2)/(sinx+xcosx)^2
参数方程是
x=tsint+2 y=2+tcost
求 dy/dx, d^2y/dx^2
x'=sint+tcost
y'=cost-tsint
x''=cost+cost-tsint=2cost-tsint
y''=-sint-sint-tcost=-2sint-tcost
dy/dt=(2+tcost)'/(tsint+2)'
=(cost-tsint)/(sint+tcost)
d^2y/dx^2=d/dx*(dy/dx)
=[(-2sint-tcost)(sint+tcost)-(cost-tsint)(2cost-tsint)]/(sint+tcost)^3
=(-2-t^2)/(sint+tcost)^3