高数-导数微分的问题，12 13，求解，谢谢啦

对于参数方程：
y=y(t)=3t-t^3
x=x(t)=2t-t^2
一阶导数：
dy/dx
=(dy/dt)/(dx/dt)
=y'/x'
=3(1-t^2) / 2(1-t)
=3(1+t)/2
那么，一阶微分：dy=3(1+t)/2 dx
二阶导数：
d^2/dx^2
=d(dy/dx)/dx
=d(y'/x')/dx
=[d(y'/x')/dt] / [dx/dt]
=[(y''x'-y'x'')/x'^2] / [x']
=(y''x'-y'x'') / x'^3
=[(-6t)(2-2t)-(3-3t^2)(-2)] / (2-2t)^3
=(-12t+12t^2+6-6t^2) / 8(1-t)^3
=(6t^2-12t+6) / 8(1-t)^3
=6(1-t)^2 / 8(1-t)^3
=3/[4(1-t)]
那么，二阶微分：d^2y=3/[4(1-t)] dx^2
不用谢我了！

(12)
x=2t-t^2
dx/dt = 2-2t

y=3t-t^3
dy/dt = 3- 3t^2

dy/dx = dy/dt / (dx/dt)
= (3-3t^2)/(2-2t)
= (3/2)(1+t)
d^2y/dt^2 = d/dt ( dy/dx) / (dy/dt)
= (3/2)/ (3- 3t^2)
= 1/[2(1-t^2)]
(13)
x+y= tany
dx +dy = (secy)^2 dy
dy= dx/(tanx)^2