Đáp án:
Ta có :
`A = 1.2.3 + 2.3.4 +3.4.5 + .... + 90.91.92 + 91.92.93`
`=>4A = 1.2.3.4 + 2.3.4.4 + 3.4.5.4+ .... + 90.91.92.4+ 91.92.93.4`
`=> 4A = 1.2.3.4 + 2.3.4.(5 - 1) +3.4.5.(6 - 2) + .... + 90.91.92.(93 - 89) + 91.92.93.(94 - 90)`
`=> 4A = 1.2.3.4 + 2.3.4.5 - 1.2.3.4 + 3.4.5.6 - 2.3.4.5 + .... + 90.91.92.93 - 89.90.91.92 + 91.92.93.94 - 90.91.92.93`
`=> 4A = (1.2.3.4 + 2.3.4.5 + 3.4.5.6 + .... + 90.91.92.93 + 91.92.93.94) - (1.2.3.4 + 2.3.4.5 + .... + 98.90.91.92 + 90.91.92.93)`
`=> 4A = 91.92.93.94`
`=> A = (91.92.93.94)/4 = 91.23.93.94`
Giải thích các bước giải:
