Are the following functions even?
f : x \longmapsto \dfrac{x\left(1-x^2\right)}{1+x}
f : x \longmapsto \dfrac{x^2 + 4}{x^4 + x^2 + 3}
f : x \longmapsto \dfrac{\left(x+3\right)\left(x-2\right)}{x^2}
f : x \longmapsto \left(x^4-1\right)\left(x-2\right)^2
f : x \longmapsto x^4 - x^2 - 2
f : x \longmapsto \dfrac{x^6 + 4x^2 - 2}{x^4 + 17}
f : x \longmapsto \dfrac{x^6 + 4x^2 - 2x}{x^4 + x^2}