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222-9+11+12:2*14+14 = ? ( )

Toán Lớp 5: Câu 14. Tính nhanh : 1/1????3+ 1/3????5+ 1/5????7+⋯+ 1/19????21

Tháng Sáu 22, 2022 Toán học 2 Comments 0 views

Toán Lớp 5: Câu 14. Tính nhanh : 1/1????3+ 1/3????5+ 1/5????7+⋯+ 1/19????21

Comments ( 2 )

  1. tonhu12
    tonhu12
    22 Tháng Sáu, 2022 at 1:15 sáng
    Reply
    1/(1xx3) + 1/(3xx5) + 1/(5xx7) + … + 1/(19xx21)
    Đặt S = 1/(1xx3) + 1/(3xx5) + 1/(5xx7) + … + 1/(19xx21)
    S xx 2 = 2 xx ( 1/(1xx3) + 1/(3xx5) + 1/(5xx7) + … + 1/(19xx21) )
    S xx 2 = 2/(1xx3) + 2/(3xx5) + 2/(5xx7) + … 2/(19xx21)
    S xx 2 = 1/1 – 1/3 + 1/3 – 1/5 + ……….. + 1/19 – 1/21
    S xx 2 = 1 – 1/21
    S xx 2 = 20/21
    S = 20/21 : 2
    S = 10/21
     

  2. truonganhthi
    truonganhthi
    22 Tháng Sáu, 2022 at 1:14 sáng
    Reply
    1/(1xx3)+1/(3xx5)+…+1/(19xx21)
    =2(1/(1xx3)+1/(3xx5)+…+1/(19xx21)):2
    =(2/(1xx3)+2/(3xx5)+…+2/(19xx21)):2
    =(1/1-1/3+1/3-1/5+…+1/19-1/21):2
    ={[(-1/3)+1/3]+[(-1/5)+1/5]+…+[(-1/19)+1/19]+1/1-1/21}:2
    ={0+0+0+…+0+20/21}:2
    =20/21 :2
    =20/21 . 1/2
    =10/21

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222-9+11+12:2*14+14 = ? ( )

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