[離散][四版習題本] ch2 關係與函數 2-65 p123

題目如下
Use the fact every polynomial equation having real-number coefficients and
odd degree has a real root in order to show that the function f:R->R, defined by
f(x) = x^5 - 2x^2 + x, is an onto function. Is f one-to-one?

我的問題是題目給的odd degree 有何用意
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