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even function

A function whose graph mirrors across the $y$ -axis — $f(-x)=f(x)$ .

A function $f$ is even if $f(-x)=f(x)$ for every $x$ in its domain, so its graph is symmetric about the $y$ -axis. Sums of only-even-power terms are even.

Examples

$f(x)=x^2$ is even — $f(-3)=f(3)=9$ .