Question

A = { ×€ Z* / 21 pe 2× +3 € Z }

Answer 1

21 pe 2x+3 apartine lui Z=> 2x+3 apartine multimii divizorilor lui 21=>

=>2x+3 apartine {-21,-7, -3, -1, 1, 3, 7, 21}=>
=>2x apartine (scadem 3 din fiecare termen (metoda balantei)) {-24, -10, -6, -4, -2, 0, 4, 18}=> x apartine (impartim la 2 fiecare termen) {-12,-5,-3 -2,-1,0,2,9}=>
=>A={-12, -5, -3, -2,-1, 0, 3, 9}