偶函数f(x)=f(-x) f(x+2)=f(-(x+2))=f(-x-2) 得证
f(x+2) = f(-x-2) ？
试转换下 令t=x+2就是了偶函数f(t)=f(-t)=f(-x-2)
偶函数：f(x+2) = f(-(x+2))
