f(x+5)=-f(x)
f(x+5+5)=f(x+2*5)=-(-f(x))=f(x)
f(x+5*n)=(-1)^n f(x) -1的n次方乘以f(x)
f(2006)=f(1+401*5)=(-1)^401*f(1)=-f(1)
f(1)=-f(-1)=1
f(2006)=-1