Đặt A = 1.2 + 2.3 + 3.4 + 4.5 + 5.6 + ....+ 999.1000
3A = 1.2.3 + 2.3.3 + 3.4.3 + 4.5.3 + 5.6.3 + .....+ 999.1000.3
3A = 1.2.( 3-0 ) + 2.3. ( 4-1 ) + 3.4. ( 5 - 2 ) + 4.5. ( 6 - 3 ) + 5.6. ( 7 - 4 ) +.....+ 999.1000. ( 1001 - 998 )
3A = 1.2.3 - 0.1.2 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4 + 4.5.6 - 3.4.5 + 5.6.7 - 4.5.6 + ....+ 999. 1000.1001 - 998.999.1000
3A = 998.999.1000 - 0.1.2
3A = 998.999.1000
A = 998.999.1000 : 3
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