### easy by hand but difficult with SageMath

Hi

Easy by hand but difficult (for me !) with SageMath
how to get this equation `dth==X*dty/r^2-Y*dtx/r^2`

from the code below with SageMath ?

```
var('theta,X,Y,r')
var('dth',latex_name=r"\partial {\theta}")
var('dtx',latex_name=r"\partial {x}")
var('dty',latex_name=r"\partial {y}")
eqL=[]
eq0=r^2==X^2+Y^2
theta=atan(Y/X)
dthX=derivative(theta,X)*dtx
dthY=derivative(theta,Y)*dty
eq1=dth==dthX+dthY
show("dthY : \t ", dthY," \t dthX : \t ",dthX," \t dth : \t ",eq1)
```