Question

[Nguyễn Xuân Cường]

Tính giá trị dãy số:1.99 + 2.98 + 3.98 + ... + 99.1

Answer 1

[Nguyễn Thị Thương Hoài]

Chưa ai giúp em sao? để  oml giúp em nhé:

A = 1.99 + 2.98 + 3.97 +......+ 99.1

A = 1(100- 1) + 2.(100-2) + 3.(100-3)+.....+99.(100-99)

A = 100 -1.1 + 2.100 - 2.2 + 3.100 -3.3 +.......+99.100 - 99.99

A =100.( 1 +2+3+....+99) - (1.1 +2.2 +3.3 +.......+99.99)

A = 100.(1+2+...+99) - { 1.(2-1) +2.(3-1) +....+99.(100-1)}

A = 100.(1+2+...+99) -{ 1.2 -1 +2.3 -2+....+99.100 -99}

A = 100.(1+2+...+99)- { -(1+2 +...+99) + (1.2 +2.3+...+99.100)

A = 100.(1+2+..+99) + (1+2+...+99) -(1.2+2.3+...99.100)

A =101.(1+2+...+99) - { 1.2.3 + 2.3.3 +....+99.100)/3

A=101.(1+99).99/ 2- {1.2.3+2.3.(4-1)+3.4.(5-2)+....+99.100.(101-1)/3}

A = 99.100.101/2 - 99.100.101/3

A =99.100.101.(1/2-1/3)

A = 99.100.101. \(\dfrac{1}{6}\)

A = 166650