Đặt A = \(\frac{4}{3.7}+\frac{4}{7.11}+\frac{4}{11.15}+...+\frac{4}{103.107}+\frac{4}{107.111}\)
\(=\left(\frac{1}{3}-\frac{1}{7}\right)+\left(\frac{1}{7}-\frac{1}{11}\right)+\left(\frac{1}{11}-\frac{1}{15}\right)+...+\left(\frac{1}{103}-\frac{1}{107}\right)+\left(\frac{1}{107}-\frac{1}{111}\right)\)
\(=\frac{1}{3}-\frac{1}{111}=\frac{37-1}{111}=\frac{36}{111}\)
Tính nhanh:A= 4/3.7+4/7.11+4/11.15+...+4/107.111
B = 4/3.7 + 4/7.11 + ....+ 4/107.111
tính tổng sau : \(A=\frac{4}{3.7}+\frac{4}{7.11}+\frac{4}{11.15}+...+\frac{4}{107.111}\)
Tính nhanh
A=\(\frac{4}{3.7}+\frac{4}{7.11}+\frac{4}{11.15}+...+\frac{1}{107.111}\)
tính các tổng sau bằng cách hợp lí
\(A=\dfrac{1}{1.2}+\dfrac{1}{2.3}+\dfrac{1}{3.4}+....+\dfrac{1}{2021.2022}\)
\(B=\dfrac{4}{3.7}+\dfrac{4}{7.11}+\dfrac{4}{11.15}+......+\dfrac{4}{107.111}\)
\(S=\dfrac{1}{31}+\dfrac{1}{32}+\dfrac{1}{33}+.....+\dfrac{1}{60}\)
a) A= 4/3.7 + 4/7.11 + 4/11.15 +...+ 4/107.111
b) B= 2/15 + 2/35 +2/63+...+ 2/399
c) C= 1/10 + 1/15 + 1/21+ ...+ 1/120
Tính hợp lí
A=\(\frac{1}{3.7}+\frac{1}{7.11}+\frac{1}{11.15}+...+\frac{1}{107.111}\)
tính nhanh tổng sau:(4/3.7)+(4/7.11)+...+(4/23.27)
a) Tìm n thuộc Z để: M = 6n - 1/3n + 2 co gia tri nguyen
b) Tinh tong A = 1/3.7 + 1/7.11 + 1/11.15 +...+ 1/107.111
