Answer:
a)  ,
,  , b)
, b)  ,
,  , c)
, c)  ,
,  .
.
Step-by-step explanation:
The equation of the circle is:

After some algebraic and trigonometric handling:


Where:


Finally,


a)  ,
,  .
.
b)  ,
,  .
.
c)  ,
, 
Where:


The solution is 
The parametric equations are:


 
        
             
        
        
        
I don't think any of them exist. the only ones I can think of make my feel there are any others
        
             
        
        
        
General equation of a circle with centre (h, k) is given by:

Now, the origin is the centre and radius is 20, so substituting these points in yields:

 
 
        
        
        
Answer:
x=-36
Step-by-step explanation:
Add the 7/2x +1/2x to be 8/2x which means that it'll be 4x=18=9/2
then subtract 9/2x on both sides so itll be 

-9/2x       -9/2x
 because if you do 8/2-9/2 it's -1/2
   because if you do 8/2-9/2 it's -1/2
then multiply the reciprocal to both sides so

which then cancels out -1/2 and makes it 
x=-36