A= 1/1.2.3+1/2.3.4+.....+1/98.99.100
2A = 2/1.2.3+ 2/2.3.4+....+2/98.99.100
Ta có: 2/1.2.3 = 1/1.2 - 1/2.3
Tương tự: 2/2.3.4 = 1/2.3 - 1/3.4
...........2/98.99.100 = 1/98.99 - 1/99.100
2A = 1/1.2 - 1/99.100 = 4949/9900
Vậy A = 4949/9900 : 2 = 4949/19800
Tích nha?
tính hợp lí: A= 1/1.2.3+1/2.3.4+...+1/98.99.100
Tính A = 1/1.2.3 + 1/2.3.4 + 1/3.4.5 + ............. + 1/ 98.99.100
Cho A=\(\dfrac{1}{1.2.3}+\dfrac{1}{2.3.4}+...+\dfrac{1}{98.99.100}\)
Chứng minh A<2
1/1.2.3 + 1/2.3.4 + .... + 1/98.99.100
tìm y biết :
a) (1/1.2.3+1/2.3.4+1/3.4.5+...+1/98.99.100)
1/1.2.3 + 1/2.3.4 + 1/3.4.5+...+1/98.99.100 = ?
1/1.2.3 + 1/2.3.4 + 1/3.4.5 + ...+1/98.99.100 =?
Tính tổng: 1/1.2.3+1/2.3.4|+...+1/98.99.100
Tính
1/ 1.2.3+ 1/ 2.3.4+ ....... + 1/ 98.99.100
