Đặt S = 1/1.2.3 - 1/2.3.4 - 1/3.4.5 - ...- 1/97.98.99
S x 2 = 2/1.2.3 - 2/2.3.4 - 2/3.4.5 - ...- 2/97.98.99
= (1/1.2 -1/2.3) - (1/2.3 - 1/3.4 ) - (1/3.4 - 1/4.5) - ...- (1/97.98 - 1/98.99)
= 1/1.2 - 1/2.3 - 1/2.3 + 1/3.4 - 1/3.4 + 1/4.5 - ....- 1/97.98 + 1/98.99
= 1/2 -1/3 + 1/98.99
= 1618/9072 => S = 1618/9072 : 2 = 809/9072
Tính 1/1.2.3-1/2.3.4-1/3.4.5-.......-1/97.98.99
Tính S=1.2.3+2.3.4+3.4.5+...+97.98.99
\(E=1.2.3+2.3.4+3.4.5+...+97.98.99\)
Tính giá trị biểu thức \(A=\frac{1}{1.2.3}+\frac{1}{2.3.4}+\frac{1}{3.4.5}+...+\frac{1}{97.98.99}+\frac{1}{98.99.100}\) .Khi đó : \(A=?\)
Tính
\(\frac{1}{1.2.3}-\frac{1}{2.3.4}-........-\frac{1}{97.98.99}\)
Tính :
\(\frac{1}{1.2.3}-\frac{1}{2.3.4}-.......-\frac{1}{97.98.99}\)
Tính: 1/1.2.3+1/2.3.4+1/3.4.5+...+1/99.100.101
Tính nhanh tổng sau: 1/1.2.3+1/2.3.4+1/3.4.5+...+1/10.11.12
Tính B= 1/1.2.3 +1/2.3.4 + 1/3.4.5 +.....+1/98/99/100
