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1/(3x5)+1/(5x7)+1/(7x9)+1/(9x11)
= (1/3-1/5 + 1/5 -1/7 + 1/7 -1/9 + 1/9 -1/11) / 2
= (1/3 - 1/11) / 2
= (11/33 - 3/33) / 2
= 4 / 33
= (1/3-1/5 + 1/5 -1/7 + 1/7 -1/9 + 1/9 -1/11) / 2
= (1/3 - 1/11) / 2
= (11/33 - 3/33) / 2
= 4 / 33
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