现在有八枚银币abcdefg，已知其中一枚是假币，其重量不同于真币，但不知道是轻还是重，如何用天平以最小的比较次数决定出那个是假币，并得知假币是比真币轻还是重。 -There are eight coins abcdefg, known to one of them is counterfeit money, and its weight is different from the real coins, but I do not know is the light or heavy, how