3.Vy∈R,F(-∞,y)=0, x∈Rl,F(x,-∞)=0, F(-∞,-∞)=0,F(∞,∞)=1 其中 F(-∞,y):=limF(x,y), F(x2∞):=limF(x,y), F(-∞2-∞):=limF(x,y), F(∞,∞):=imF(x,y)3. yR 1, F(-∞,y)=0， xR 1, F(x,-∞)=0， F(-∞,-∞)=0， F(∞,∞)=1 ( , ): lim ( , ) ( , ): lim ( , ) , ( , ): lim ( , ), ( , ): lim ( , ) , F F x y F F x y F x F x y F y F x y x x y x y x → → →− →− →− →−   = − − = − = − = 其中: