Our experience shows that customers will sometimes buy ___1____, sometimes order on the ____2______, and sometimes go into the stores to pick up goods. So we need to understand what our customers want.
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【简答题】若对任意x∈R,y∈R有唯一确定的f (x,y)与之对应,则称f (x,y)为关于x,y的二元函数.定义:同时满足下列性质的二元函数f (x,y)为关于实数x,y的广义“距离”: (Ⅰ)非负性:f (x,y)≥0; (Ⅱ)对称性:f (x,y)= f (y,x); (Ⅲ)三角形不等式:f (x,y)≤f (x,z)+ f (z,y)对任意的实数z均成立. 给出下列二元函数: ①f (x,y)=(x...