Which of the following statements below is not trye about rational numbers? a)the product of two rational numbers is always irrational. b)the sum of a rational number and an irrational number is always irrational. c)the product of a nonzero rational number and an irrational number is always irrational. d)the product of two irrational numbers is either rational or irrational.

not only in mathematics, scientific proof also are required in the other field of science. also maths proofs are of particular structure, containing steps and logics. so proving is like doing science, it is science. that's why it is important to prove. 

an inaccurate proof leads to errors, or stating results that are not accurate.  which wil mess the probelm up you dont want this

cross multiplying, we get :

⇒ (3) × (5) = (9) × (x)

⇒ 15 = 9x