10x = 9.9999999.... (1)
x = 0.99999999..... (2)
(1) - (2) : 9x = 9
x = 1
จาก (2) : x = 0.99999999.....
-------------------------------------------------
1/ 3 = 0.333333333.... (1)
2/3 = 0.666666666.... (2)
(1) + (2) : 1/3 + 2/3 = 0.3333333... + 0.66666666.....
1 = 0.999999999999
สรุป : 0.999999999 = 1
Reference : http://polymathematics.typepad.com/
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