若函数f(x),g(x)满足 g(x-y)=g(x)g(y)+f(x)f(y),并且f(0)=0,f(-1)=-1,f(1)=1.
(1)证明:f2(x)+g2(x)=g(0).
(2)求g(0),g(1),g(-1),g(2)的值.
(3)判断f(x),g(x)的奇偶性.