
Answer:
a) q ≠0 ,  b)  r .s = p1 / q1. p2 / q2  = p3/q3,  c)
Explanation:
A rational number is a number of the form p / q where the p and q values ​​are integers,
Where we assume that q is different from zero
q ≠0
Since the division by zero is not defined
This is the only assumption to be made.
b) r .s = p1 / q1. p2 / q2
   If p1 and p2 are integers your product is another integer
     P1. p2 = p3
If q1 and q2 are integer your product is integer, none can be zero
    q1. q2 = q3
     p3 / q3 = r3
 What is a rational, what proves the theorem
c) the sum of two rational is another rational
      2 + y = r3
 Let's write the numbers with rational
     2 = p / q y = p2 / q2
      p / q + p2 / q2 = p3 / q3
       q = 1
       p = 2
      (2q2 + p2) / q2 = p3 / q3
We see that the numerator and denominator are true for which the theorem is true