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

Toán Lớp 7: Tính `A = 1.2 + 2.3 + 3.4 + … + n.(n + 1)`

Toán Lớp 7: Tính A = 1.2 + 2.3 + 3.4 + … + n.(n + 1)

Comments ( 1 )

  1. Lời giải và giải thích chi tiết:
    A = 1 . 2 + 2 . 3 + 3 . 4 + ………… + n . (n + 1)
    3A = 1 . 2 . 3 + 2 . 3 . 3 + 3 . 4 . 3 + ……… + n . (n + 1) . 3
    3A = 1 . 2 . 3 + 2 . 3 . (4 – 1) + 3 . 4 . (5 – 2) + ……….. + n . (n + 1) . [(n + 2) – (n – 1)]
    3A = 1 . 2 . 3 + 2 . 3 . 4 – 1 . 2 . 3 + 3 . 4 . 5 – 2 . 3 . 4 + …………. + n . (n + 1) . (n + 2) – (n – 1) . n . (n + 1)
    3A = n . (n + 1) . (n + 2)
    A = (n . (n + 1) . (n + 2))/3

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