By the theory of Pell's equation, we give conditions for f (x) = b + c x with b, c ∈ Z\{0} such that the Diophantine equations z 2 = f (x) 2 ± f (y) 2 have infinitely many solutions x, y ∈ Z and z ∈ Q, which gives a positive answer to Question 3.2 of Zhang and Shamsi Zargar [16]. By the theory of elliptic curve, we study the non-trivial rational solutions of the above Diophantine equations for Laurent polynomials f, and give a positive answer to Question 3.1 of Zhang and Shamsi Zargar [16].