3\51+4\51+6\51+...+58\51-1\51-3\51-5\51-...49\51中第一个3应当是2.58应当是50 即2\51+4\51+6\51+...+50\51-1\51-3\51-5\51-...49\51=(2/51-1/51)+(4/51-3/51)--+(50/51-49/51)=1/51+1/51----+1/51(共25个）=25/51...