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

Toán Lớp 5: A= 5/1 + 5/3 + 5/9 + 5/27 + 5/81 + 5/243 tinh nhanh nha

Toán Lớp 5: A= 5/1 + 5/3 + 5/9 + 5/27 + 5/81 + 5/243
tinh nhanh nha

Comments ( 2 )

  1. A=5/1+5/3+5/9+5/27+5/81+5/243
    ⇒A/3=5/3+5/9+5/27+5/81+5/243+5/729
    ⇒A−A/3=(5/1+5/3+5/9+5/27+5/81+5/243)−(5/3+5/9+5/27+5/81+5/243+5/729)
    ⇒(2A)/3=5−5/729
    ⇒(2A)/3=3640/729
    ⇒2A=3640/729×3
    ⇒2A=3640/243
    ⇒A=3640/243:2
    ⇒A=1820/243
    Vậy A=1820/243

  2. đáp án :
    a = 5/1 + 5/3 + 5/9 + 5/27 + 5/81 + 5/243 
    = ( 5/1 + 5/243 ) + ( 5/9 + 5/81 ) + ( 5/3+5/27)
    = 1820 / 243
     

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

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