Even function is f(x)=f(-x).

For example: f(x)=x? When x= 1, f(x)=f( 1)= 1? = 1

When x=- 1, f(x)=f(- 1)=(- 1)? = 1, that is, f(x)=f(-x)

f(x)=x？ When x= 1, f(x)=f( 1)= 1? = 1

When x=- 1, f(x)=f(- 1)=(- 1)? =- 1, that is, f(x)=-f(x)

I don't know how to explain this, but can I explain it to you clearly?