Which of the following expressions are polynomials? Justify your answer:
(i) 8
(ii) 3x^2 −2x
(iii) 1− 5x (iv) 1/5x^−2 +5x+7
(v) (x−2)(x−4)/x​
(vi) 1/x+1 (vii) 1/7 a^3 − 1/3 a^2 +4a−7
(viii) 1/2x