數學題:我要方法(快,簡單)

1. 項數=(2005-1)/÷2+1=1003
把 (1-3)、(5-7) 為視為一組，組別數目=(1003-1)÷2=501
即是 (-2)×501=(-1002)
答案=2005+(-1002)=1003

2.1989*17891789-1789*18991899
=1989×1789×10001-1789×1899×10001
=0

2.123123123/193193193+456456456/193193193
=123123123/193193193+123123123×3/193193193
=123123123×4/193193193
=123×1001001×4/193×1001001
=123×4/193
=492/193

2007-05-14 21:13:00 補充：
第三題錯了 ==3. 123123123/193193193 456456456/193193193=123×1001001/193×1001001 456×1001001/193×1001001=123/193 456/193=(123 456)/193=579/193=3

2007-05-14 21:15:03 補充：
沒了＋號==123123123/193193193＋456456456/193193193=123×1001001/193×1001001＋456×1001001/193×1001001=123/193＋456/193=(123＋456)/193=579/193=3

good