Maths~~~~help~
回答 (3)
2007-09-30 12:34 am
✔ 最佳答案
1-2+3-4+5.....+99-100=1+3+5+7+....+99-2-4-6-8-....-100
=1+3+5+7+....+99-(2+4+6+8+...+100)
=(50/2)(1+99)-(50/2)(2+100)
=2500-2550
=-50
2007-09-29 16:38:37 補充：
亦都可以咁睇(1-2)+(3-4)+(5-6)+.....+(97-98)+(99-100)每一組( )都等於-1.........總共有50組因此=-50
2007-09-29 16:42:17 補充：
等二步去第三步係用公式總和=(頭項加尾項)x項數/2
2007-09-30 12:57 am
1-2+3-4+5.....+99-100=150
2007-09-30 12:44 am
so, you can see that every "-" as a group, so, 1-2 is one group and 3-4 is another group. So, every answer is -1. There are 50 groups altogther (becuase 100/2 = 50). So, you can use -1 x 50 = -50.
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