f(-x) flips over Y-axis. -f(x) flips over X-axis. -f(-x) flips over both X-axis and Y-axis; equivalent to 180° rotation about origin.
If f(x)=f(-x), then f is Even (symmetric across the Y-axis). Also: -f(x)=-f(-x) If f(-x)=-f(x), then f is Odd (symmetric about the Origin). Also: f(x)=-f(-x)