giúp mk với ạ 1) 21/20+31/30+43/42+…+111/110+133/132 2) 1/(11×13)+1/(13×15)+1/(15×18)+…+1/(75×77)+1/(77×79)

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1.  1.

   21/20 + 31/30 + 43/42 +…..+ 111/110 + 133/132
   = (1 + 1/20) + (1 + 1/30) + (1 + 1/42) + …+ (1 + 1/110) + (1 + 1/132)
   = (1 + 1 + 1 + …+ 1) + (1/20 + 1/30 + 1/42 + ….+ 1/110 + 1/132)
   = 8 + (1/4.5 + 1/5.6 + 1/6.7 + …+ 1/10.11 + 1/11.12)
   = 8 + (1/4 – 1/5 + 1/5 – 1/6 + 1/6 – 1/7 + …+ 1/10 – 1/11 + 1/11 – 1/12)
   = 8 + (1/4 – 1/12)
   = 8 + 1/6
   = 49/6

   2.1/(11×13)+1/(13×15)+1/(15×18)+…+1/(75×77)+1/(77×79)

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