-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.