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

Toán Lớp 9: `a) A=3-3^2+3^3-3^4+-…+3^2019-3^2020 `

Tháng Chín 5, 2022 Toán học 1 Comment 0 views

Toán Lớp 9: a) A=3-3^2+3^3-3^4+…….+3^2019-3^2020 b) B=1-3+5-7+9-11+……+2013-2015

Comments ( 1 )

  1. lyly98
    lyly98
    5 Tháng Chín, 2022 at 9:39 sáng
    Reply
    a) A = 3 – 3^2 + 3^3 – 3^4 + …. + 3^{2019} – 3^{2020}
    => 3A = 3^2 – 3^3 + 3^4 – 3^5 + …. + 3^{2020} – 3^{2021}
    => 3A + A = 3^2 – 3^{2021} + 3 – 3^{2}
    => 4A = 9 – 3^{2021} + 3 – 9
    => 4A = 3^{2021} + (-9) + 3 – 9
    => 4A = 3^{2021} + (-6) – 9
    => 4A = 3^{2021} + (-15)
    => A = [ 3^{2021} + (-15) ] : 4
    b) B = 1 – 3 + 5 – 7 + 9 – 11 + ….  + 2013 – 2015
    => B = ( 1 – 3 ) + ( 5 – 7 )  + ( 9 – 11 ) + ….  + ( 2013 – 2015 )
    => B = ( -2 ) + ( -2 ) + ( -2 ) + …. + ( -2)
    => B = ( -2 ) . { [ ( 2015 – 1 ) : 2 + 1 ] : 2 }
    => B = ( -2 ) . 504
    => B = -1008

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

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