Symmetry


  Plot[Cos[x],{x,-2 Pi,2 Pi},AspectRatio->Automatic];


  Plot[Sin[x],{x,-2 Pi,2 Pi},AspectRatio->Automatic];

The graph of an even function is symmetric with respect to the y axis.
The graph of an odd function is symmetric with respect to the origin.

Look at the graphs of x^k.


  p=Table[Plot[x^k,{x,-2,2},DisplayFunction->Identity],{k,1,6}];


  Show[GraphicsArray[{Table[p[[k]],{k,1,2}],
   Table[p[[k]],{k,3,4}],Table[p[[k]],{k,5,6}]}]];