f(x+2)=-f(x-1)令t=x-1,x=t+1,x+2=t+3,f(t+3)=-f(t)，一样的办法,f(x+6)=-f(x+3),-f(x+3)=f(x)得到f(x+6)=f(x)
f(x+2)=-f(x-1),说明周期T=3,故f(x+6)=f(x+3)=f(x)
