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

Toán Lớp 5: tìm x 1/15+1/35+…+1/X x (x +2) = 1/7 (làm trắc nghiệm nhé)

Home/ Toán Lớp 5: tìm x 1/15+1/35+…+1/X x (x +2) = 1/7 (làm trắc nghiệm nhé)

Toán Lớp 5: tìm x 1/15+1/35+…+1/X x (x +2) = 1/7 (làm trắc nghiệm nhé)

Tháng Chín 6, 2022 Toán học 2 Comments 0 views

Toán Lớp 5: tìm x 1/15+1/35+…+1/X x (x +2) = 1/7 (làm trắc nghiệm nhé)

Comments ( 2 )

  1. thanhbinh2k5
    thanhbinh2k5
    6 Tháng Chín, 2022 at 9:30 sáng
    Reply
    Xin hay nhất nhé tus ^_^
    $\dfrac{1}{15}$ + $\dfrac{1}{35}$ +…+ $\dfrac{1}{x×(x+2)}$ = $\dfrac{1}{7}$
    $\dfrac{1}{2}$×($\dfrac{2}{15}$ + $\dfrac{2}{35}$ +…+ $\dfrac{2}{x×(x+2)}$) = $\dfrac{1}{7}$
    $\dfrac{1}{2}$×($\dfrac{2}{3×5}$ + $\dfrac{2}{5×7}$ +…+ $\dfrac{2}{x×(x+2)}$) = $\dfrac{1}{7}$
    $\dfrac{1}{2}$×($\dfrac{1}{3}$ – $\dfrac{1}{5}$ + $\dfrac{1}{5}$ – $\dfrac{1}{7}$ +…+ $\dfrac{1}{x}$ – $\dfrac{1}{x+2}$) = $\dfrac{1}{7}$
    $\dfrac{1}{2}$×($\dfrac{1}{3}$ – $\dfrac{1}{x+2}$) = $\dfrac{1}{7}$
    $\dfrac{1}{3}$ – $\dfrac{1}{x+2}$ = $\dfrac{1}{7}$ : $\dfrac{1}{2}$
    $\dfrac{1}{3}$ – $\dfrac{1}{x+2}$ = $\dfrac{2}{7}$
    $\dfrac{1}{x+2}$ = $\dfrac{1}{3}$ – $\dfrac{2}{7}$
    $\dfrac{1}{x+2}$ = $\dfrac{1}{21}$
    ⇒ x+2=21
        x=21-2
        x=19
    Vậy x=19

  2. giahan44
    giahan44
    6 Tháng Chín, 2022 at 9:29 sáng
    Reply
    Ta có:
    2xx(1/15+1/35+….+1/{x.(x+2)})=1/7
    2xx(2/15+2/35+…+2/{x.(x+2)})=1/7
    2xx(1/3xx5+1/5xx7+…+1/{x-(x+2)})=1/7
    2xx(1/3-1/x+2)=1/7
    2xx(1/x+2)=1/3-1/7
    2xx(1/x+2)=4/21
    1/x+2=4/21:2
    1/x+2=1/21
    =>x+2=21
    =>x=21-2
    =>x=19
    Vậy x=19

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

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