奇函数即是 f(-x)= -f(x)
f(x+2) = f(x) -1
令X= - 1
即
f( - 1 + 2) = f( - 1 ) - 1===> f(1)=f( -1 ) - 1 ===> f(1) = - f(1) - 1==> f(1) = - 0.5