a)6/1.4+6/4.7+6/7.10+-+6/97.100

b 4/1.3+16/3.5+36/5.7+-.+9604/97.99

c (1/1.2+1/3.4+1/5.6+--...+1/19.20)-(1/11+1/12+-.+1/20)

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