1 câu trả lời
Đặt A = $1.2 + 2.3 + 3.4 + ....+ 101.102$
Ta có :
A = $1.2 + 2.3 + 3.4 + ....+ 101.102$
=> 3A = $1.2.3 + 2.3.3 + 3.4.3 + .... + 101.102.3$
=> 3A = $1.2.3 + 2.3.(4-1) + 3.4.(5-2) + .... + 101.102.(103-100)$
=> 3A = $1.2.3 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4 + .... + 101.102.103 - 100.101.102$
=> 3A = $101.102.103$
=> A = $101 . 102 . 103 / 3$
=> A = $101 . 34 . 103$
=> A = $353702$
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