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Math Nerdery Thread

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My mother is a math professor, and showed me this proof after seeing it at a conference:


...99999999.0 represents an infinitely large number composed of 9's and stopping at the decimal place)


...99999999.0 = x

...99999990.0 = 10x (as the 9's will stretch to infinity all the same, even when multiplied by 10)

                      9 = -9x

                     -1 = x

...99999999.0 = -1


Secondary way of looking at it:


...9999999999.0 = x

(add 1)

...0000000000.0 = x+1 (as the carried-over one from adding one to 9 will never drop down into the number)

...9999999999.0+1 = 0

...9999999999.0 = -1

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