is to divide the front 1 / n ^ 3 into three 1zones, which are multiplied by the three factors of the following molecules. For example,

$$ \ frac {1} {n} * (nMur1) = 1 -\ frac {1} {n} $$

learn how to write the formula, the train of thought is upstairs

$$ \ frac {1} {n ^ 3} *\ frac {(nmur1) n (2n-1)} {6} =\ frac {(n Mei 1) n (2n-1)} {6* n ^ 3} $$

$$ =\ frac {1} {3} *\ frac {n} *\ frac {n} {n} *\ frac {2n-1} {2n} =\ frac {1} {3} * (1 -\ frac {1} {n}) * 1* (1 -\ frac {1} {2n}) $$

$$ =\ frac {1} {3} * (1 -\ frac {1} {n}) * (1 -\ frac {1} {2n}) $$