11.13.3 Can you answer these questions?

11.13.3.1 Avoid Simplify introducing singularities when using trigonometric identities

https://mathematica.stackexchange.com/landing/r/digest?cta=question&id=312153

 Simplify  (and  FullSimplify ) can use trigonometric identities in a way that introduces singularities when ... 
[calculus-and-analysis] [simplifying-expressions] [assumptions] [trigonometry]
 

asked by ZehDeckel https://mathematica.stackexchange.com/landing/r/digest?cta=user&id=93027 Score of 3

11.13.3.2 Precision/Accuracy error in solving numerical equation with FindRoot

https://mathematica.stackexchange.com/landing/r/digest?cta=question&id=312101

I am trying to solve the equation bellow  eq[x,k]  for  x , for  k  from ... 
[equation-solving] [precision-and-accuracy]
 

asked by Gallagher https://mathematica.stackexchange.com/landing/r/digest?cta=user&id=58233 Score of 1

11.13.3.3 Can not convert from implict region to parametric region

https://mathematica.stackexchange.com/landing/r/digest?cta=question&id=312146

I can not convert implicit region to parametric region, even the simple circle. 
 
This is input code: 
... 
[regions] [implicitregion]
 

asked by GongNanyue https://mathematica.stackexchange.com/landing/r/digest?cta=user&id=90468 Score of 2