令t=-x＜0
则x=-t
f(-t) = (-t)^2-2*(-t) = t^2+2t
f(t)为奇函数
f(-t)+f(t) = 0
f(t) = -f(-t) = -t^2-2t
将t换成x,即是：当x＜0时,f(x) = -x^2-2x