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Question

Tính nhanh

1/10+1/15+1/21+.....+1/120

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Khó quá

Answer 1

1/10+1/15+1/21+.....+1/120=2/20+2/30+2/42+....+2/240=2x(1/20+1/30+1/42+.....+1/240)=2x(1/(4x5)+1/(5x6)+1/(6x7)+.....+1/(15x16)=2x(1/4-1/5+1/5-1/6+1/6-..........+1/15-1/16)=2x(1/4-1/16)=3/8

Answer 2

Đặt S = 1/10 + 1/15 + ... + 1/120

Ta có :

S = 1/10 + 1/15 + ... + 1/120

S = 2/20 + 2/30 + ... + 2/240

S = 2.(1/20 + 1/30 + ... + 1/240)

S = 2.(1/4.5 + 1/5.6 + ... + 1/15.16)

S = 2.(1/4 - 1/5 + 1/5 - 1/6 + ... + 1/15 - 1/16)

S = 2.(1/4 - 1/16)

S = 2. 3/16

S = 3/8

Answer 3

Đặt A = 1/10 + 1/15 + ... + 1/120

Ta có :

A = 1/10 + 1/15 + ... + 1/120

A = 2/20 + 2/30 + ... + 2/240

A = 2.(1/20 + 1/30 + ... + 1/240)

A = 2.(1/4.5 + 1/5.6 + ... + 1/15.16)

A = 2.(1/4 - 1/5 + 1/5 - 1/6 + ... +1/15 - 1/16)

A = 2.(1/4 - 1/16)

A = 2 . 3/16

A = 3/8