Question

tính nhanh

201.3+2/3.5+2/5.7+...+2/199.201

Answer 1

ta có:

201.3 + 2/3.5+2/5.7+..................+2/199.201

= 201.3+ 2/2 . ( 1/3 -1/5)+2/2.(1/5-1/7)+2/2.(1/5-1/7) +..............+2/2.(1/199.1/201)

201.3 +2/2.1/3.-2/2.1/5+2/2.1/5-2/2.1/7..........................+2/2.1/199-2/2.1/201

=201.3 +2/2.(1/3+1/5-1/5+1/7-1/7.................+1/99.1/201)

=201.3+2/2.(1/3-1/201)

=201.3+22/67

=198

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