Toán D= 1.2.3+ 2.3.4 + 3.4.5+….. + n.(n+1).(n+2) ( n thuộc N sao) tính D 03/12/2021 By Camila D= 1.2.3+ 2.3.4 + 3.4.5+….. + n.(n+1).(n+2) ( n thuộc N sao) tính D
$4D = 4.[1.2.3 + 2.3.4 + 3.4.5 + … + (n – 1).n.(n + 1)]$ $4D = 1.2.3.4 + 2.3.4.4 + 3.4.5.4 + … + (n – 1).n.(n + 1).4$ $4D = 1.2.3.4 + 2.3.4.(5 – 1) + 3.4.5.(6 – 2) + … + (n – 1).n.(n + 1).[(n + 2) – (n – 2)]$ $4D = 1.2.3.4 + 2.3.4.5 – 1.2.3.4 + 3.4.5.6 – 2.3.4.5 + … + (n – 1).n(n + 1).(n + 2) – (n – 2).(n – 1).n.(n + 1)$ $4D = (n – 1).n(n + 1).(n + 2)$ $D = (n – 1).n(n + 1).(n + 2) : 4$ Trả lời
$4D = 4.[1.2.3 + 2.3.4 + 3.4.5 + … + (n – 1).n.(n + 1)]$
$4D = 1.2.3.4 + 2.3.4.4 + 3.4.5.4 + … + (n – 1).n.(n + 1).4$
$4D = 1.2.3.4 + 2.3.4.(5 – 1) + 3.4.5.(6 – 2) + … + (n – 1).n.(n + 1).[(n + 2) – (n – 2)]$
$4D = 1.2.3.4 + 2.3.4.5 – 1.2.3.4 + 3.4.5.6 – 2.3.4.5 + … + (n – 1).n(n + 1).(n + 2) – (n – 2).(n – 1).n.(n + 1)$
$4D = (n – 1).n(n + 1).(n + 2)$
$D = (n – 1).n(n + 1).(n + 2) : 4$