A because radius is half of the diameter
        
                    
             
        
        
        
Arcs are just sections of the circle 
so lets say you had a pizza
and you put some points on the crust, the legnth of the crust from point to point is that arc
example
if you put poins A and B, then arc AB is the crust between points A and B
arcs can include other points as well
 and arc AB=arc BA
so 
I wil name easy arcs first
arc AB
arc BC
arc CD
ard DA
now ones that include others
arc AC
arc BD
so the arcs you might want to list are
  
arc AB
arc BC
arc CD
ard DA
arc AC
arc DB
        
             
        
        
        
15.
To find this, here is the work in the photo attached:
 
        
        
        
Answer:
x < 1/4
Step-by-step explanation:
 
        
             
        
        
        
Answer:
P(Y=1|X=3)=0.125
Step-by-step explanation:
Given :
p(1,1)=0  
p(2,1)=0.1
p(3,1)=0.05
p(1,2)=0.05
p(2,2)=0.3
p(3,2)=0.1
p(1,3)=0.05
p(2,3)=0.1
p(3,3)=0.25 
Now we are supposed to find the conditional mass function of Y given X=3 :  P(Y=1|X=3)
 P(X=3) = P(X=3,Y=1)+P(X=3,Y=2) +P(X=3,Y=3) 
P(X=3)=p(3,1) +p(3,2) +p(3,3) 
P(X=3)=0.05+0.1+0.25=0.4

Hence P(Y=1|X=3)=0.125