數學問題the distance between a and b

the distance between a and b = a 和 b 兩點既距離
點解會係兩倍呢??
the distance between a(x1,y1) and b(x2,y2)
= sqrt[ (x1 - x2)^2 + (y1 - y2)^2 ]
sqrt[ ] 係平方根
係咪運算錯誤?? 小心再計多次