Even functions are when f(-x)=f(x) They are symmetrical about the y axis
Odd functions are when f(-x)=-f(x) The have point symmetry about the origin. This means that if f(-x) is rotated 180 degrees about (0,0) it becomes the f(x)
lets look at your one
f(x) = (x)^4 - 20* (x)^2
f(-x) = (-x)^4 - 20* (-x)^2
f(-x) = (x)^4 - 20* (x)^2 since (-x)^2 = x^2 and (-x)^4 = x^4
f(-x) = f(x)
So it is an even function
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