State whether each of the following functions is even or odd.
(i) `f(x) = x^2`
(ii) `f(x) = log (cos x)`
(iii)` f(x) = sin x`
(iv)` f(x) = log (x+sqrt(1+x^2))`
(v) `f(x) = 3`
(vi) `f(x) = x^(-5)`
(vii) `f(x) = x^2 +1`
(viii) `f(x) = x^2 + x`
(ix) `f(x) = x^4 + 3x^2 -1`
(x) `f(x) =1/(x^2-1)`
(xi) `f(x) = x/(x^2-1)`
(xii) `f(x) = | x^3|`
(xiii) `f(x) = 2 |x| + 1`