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