tính tổng:
 $T=1.2 + 2..3+3.4+..............+(n-1).n$
bài này quen thế! –  congiola_ktqd 06-01-13 10:03 PM
thank trần hoàng sơn nhìu –  lovelove15071995 05-01-13 10:43 PM
Ta có:
$3T= 1.2.(3-0)+ 2.3.(4-1)+...+ n.(n+1).[(n+2)-(n-1)]$
$=[1.2.3+ 2.3.4+...+ (n-1)n(n+1)+ n(n+1)(n+2)]- [0.1.2+ 1.2.3+...+(n-1)n(n+1)]$
$=n(n+1)(n+2)$
=> $T=\frac{n(n+1)(n+2)}{3}$
CKCC!!!! –  congiola_ktqd 06-01-13 10:03 PM
click nút V dưới đáp án nếu có hữu ích với bạn nhé –  Trần Hoàng Sơn 05-01-13 05:07 PM

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