\(\Rightarrow A=\frac{1}{2}.\left(\frac{2}{3.5}+\frac{2}{5.7}+......+\frac{2}{13.15}\right)\)
\(\Rightarrow A=\frac{1}{2}.\left(\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+............+\frac{1}{13}-\frac{1}{15}\right)\)
\(\Rightarrow A=\frac{1}{2}.\left(\frac{1}{3}-\frac{1}{15}\right)\)
\(\Rightarrow A=\frac{1}{2}.\frac{4}{15}\)
\(\Rightarrow A=\frac{2}{15}\)
Tính tổng: \(S=\frac{1}{1.3}-\frac{1}{2.4}+\frac{1}{3.5}-\frac{1}{4.6}+\frac{1}{5.7}-\frac{1}{6.8}+\frac{1}{7.9}-\frac{1}{8.10}\)
\(\dfrac{2}{3.5}+\dfrac{2}{5.7}+\dfrac{2}{7.9}+\dfrac{2}{9.11}+\dfrac{2}{11.13}+\dfrac{2}{13.15}\)
đề : tính
S =1/1.3-1/2.4+1/3.5-1/4.6+1/5.7 - 1/6.8+1/7.9-1/8.10
tìm y
[ 1/1.3+1/3.5+1/5.7+1/7.9 ] . y = 2/3
Tính:
\(A=\dfrac{1}{3.5}+\dfrac{1}{5.7}+\dfrac{1}{7.9}+...........+\dfrac{1}{99.101}\)
S= 1/3.5+1/5.7+1/7.9+...+1/97.99
(cac ban chu y: dau / la phan so va dau . la dau nhan)
Tính nhanh tổng sau
A= \(\dfrac{2}{3.5}+\dfrac{2}{5.7}+\dfrac{2}{7.9}+...+\dfrac{2}{59.61}\)
Tính các tổng sau
s= 1+4+7+...+40
p= 1/3.5 + 1/5.7 + 1/7.10 +... + 1/51.53
Tính nhanh:
S = \(\frac{1}{1.3}+\frac{1}{3.5}+\frac{1}{5.7}+\frac{1}{7.9}+.....+\frac{1}{95.97}+\frac{1}{97.99}\)
