Look at the following column: half, one sixth, one twelfth, one twentieth And so on, what are the ninth and fourteenth numbers? Why?

1/(1x2),- 1/(2x3),1/(3x4),-1/(4x5).
The rule is that the nth number is (- 1) ^ (n-1) / (NX (n + 1))
Therefore, the ninth number is 1 / (9x10) = 1 / 90, and the fourteenth number is - 1 / (14x15) = - 1 / 210

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