-if f(-x)=-f(x), it is a odd function, and if f(-x)=f(x), it is an even function;

Method 2, simplify f(-x)+f(x):

-if f(-x)+f(x)=0, it is odd function;

Method 3, simplify f(-x)-f(-x):

-If f(-x)-f(x)=0, it is an even function.