Observe the following column: 1 / 2, 1 / 2, 1 / 12, Please tell me the law and the ninth number,
1 out of 2, 1 out of 6, 1 out of 12, 1 out of 20
1/2=1/1x2
-1/6=-1/2x3
1/12=1/3x4
-1/20=1/4x5
So the ninth number is 1 / 9x10 = 1 / 90
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