1 × 3 / 1 + 3 × 5 / 1 + 5 × 7 / 1 + 7 × 9 / 1 + 9 × 11 / 1 + 11 × 13 / 1 A math problem

1 × 3 / 1 + 3 × 5 / 1 + 5 × 7 / 1 + 7 × 9 / 1 + 9 × 11 / 1 + 11 × 13 / 1
=(1-1/3)/2+(1/3-1/5)/2+(1/5-1/7)/2+(1/7-1/9)/2+(1/11-1/13)/2
=(1-1/3+1/3-1/5+1/5-...-1/13)/2
=(1-1/13)/2
=6/13

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