2/15+2/35+2/63+2/99
= 2/(3×5) + 2/(5×7) + 2/(7×9) + 2/(9×11)
= (1/3 - 1/5) + (1/5 - 1/7) + (1/7 - 1/9) + (1/9 - 1/11)
= 1/3 - 1/11
= (11-3)/33
= 8/33
2/15+2/35+2/63+2/99
=2/3-2/5+2/5-2/7+2/7-2/9+2/9-2/11
=2/3-2/11
=16/33
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