Đặt S = 1.2.3 + 2.3.4 + 3.4.5 + .... + 98.99.100
=> 4S = 1.2.3.4 + 2.3.4.4 + 3.4.5.4 + ... + 98.99.100.4
=> 4S = 1.2.3.4 + 2.3.4.( 5 - 1 ) + 3.4.5.( 6 - 2 ) + .... + 98.99.100.( 101 - 97 )
=> 4S = 1.2.3.4 + 2.3.4.5 - 1.2.3.4 + 3.4.5.6 - 2.3.4.5 + .... + 98.99.100.101 - 97.98.99.100
=> 4S = ( 1.2.3.4 - 1.2.3.4 ) + ( 2.3.4.5 - 2.3.4.5 ) + ...... + ( 97.98.99.100 - 97.98.99.100 ) + 98.99.100.101
=> 4S = 98.99.100.101
=> S = \(\frac{98.99.100.101}{4}\)
=> S = 24497550
1.2.3+2.3.4.+3.4.5+........+98.99.100 bằng?
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