Question

tính tổng 

S=1.2+2.3+............+999.1000

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Answer 1

S = 1.2 + 2.3 + 3.4 + 4.5 + ... + 999. 1000

=> 3S = 1.2.3 + 2.3.3 + 3.4.3 + ... + 999.1000.3

3S = 1.2.(3 - 0) + 2.3.(4 - 1) + 3.4.(5 - 2) + ... + 999.1000.(1001 - 998)

3S = 1.2.3 - 1.2.0 + 2.3.4 -1.2.3 + 3.4.5 - 2.3.4 + ... + 999.1000.1001 - 998 .999.1000

3S = 999.1000.1001 

=> S = (999.1000.1001) : 3 = 999999000 : 3 = 333333000

Answer 2

S = 1.2 + 2.3 + 3.4 + 4.5 + ... + 999.1000

=> 3S = 1.2.3 + 2.3.3 + 3.4.3 + ... + 999.1000.3

3S = 1.2.(3-0) + 2.3.(4-1) + 3.4(5-2) + ... + 999.1000(1001-998)

3S = 1.2.3 - 1.2.0 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4 + ... + 999.1000.1001 - 998.999.1000

3S = 999.1000.1001

=> S = (999.1000.1001) : 3 = 999999000 : 3 = 333333000

Answer 3

S = 1.2 + 2.3 + 3.4 + 4.5 + ... + 999.1000 => 3S = 1.2.3 + 2.3.3 + 3.4.3 + ... + 999.1000.3 3S = 1.2.(3-0) + 2.3.(4-1) + 3.4(5-2) + ... + 999.1000(1001-998) 3S = 1.2.3 - 1.2.0 + 2.3.4 - 1.2.3 + 3.4.5 - 2.3.4 + ... + 999.1000.1001 - 998.999.1000 3S = 999.1000.1001 => S = (999.1000.1001) : 3 = 999999000 : 3 = 333333000

Answer 4

cảm ơn ban nha