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S = 2/3x5 + 2/5x7 + 2/7x9 + .... + 2/31x33

Răspuns :

S=2/3x5+2/5x7+2/7x9+...+2/31x33=1/3-1/5+1/5-1/7+1/7-1/9+...+1/31-1/33=33-3/33=30/33=10/11