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

Toán Lớp 7: Tính 1/5.9+1/9.13+1.13.17+1/17.21+1/21.25.

Toán Lớp 7: Tính 1/5.9+1/9.13+1.13.17+1/17.21+1/21.25.

Comments ( 2 )

  1. Đặt A = 1/(5.9) + 1/(9.13) + 1/(13.17) + 1/(17.21) + 1/(21.25)
    4A = 4/(5.9) + 4/(9.13) + 4/(13.17) + 4/(17.21) + 4/(21.25)
    4A = 1/5 – 1/9 + 1/9 – 1/13 + 1/13 – 1/17+1/17-1/21 + 1/21 – 1/25
    4A = 1/5 – 1/25
    4A = 5/25 – 1/25
    4A = 4/25
    A = 4/25 : 4
    A = 4/25 . 1/4
    A = 1/25
    (Chúc bạn học tốt)

  2. Đặt:
    A = 1/5.9 + 1/9.13 + 1/13.17 + 1/17.21 + 1/21.25
    4A = 4 . (1/5.9 + 1/9.13 + 1/13.17 + 1/17.21 + 1/21.25)
    4A = 4/5.9 + 4/9.13 + 4/13.17 + 4/17.21 + 4/21.25
    4A = 1/5 – 1/9 + 1/9 – 1/13 + 1/13 – 1/17 + 1/17 – 1/21 + 1/21 – 1/25
    4A = 1/5 – 1/25
    4A = 5/25 – 1/25
    4A = 4/25
    A = 4/25 : 4
    A = 1/25

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

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