\(A=\frac{2}{15}+\frac{2}{35}+\frac{2}{63}+...+\frac{2}{399}\)
=>\(A=\frac{2}{3.5}+\frac{2}{5.7}+\frac{2}{7.9}+...+\frac{2}{19.21}\)
=>\(A=\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+...+\frac{1}{19}-\frac{1}{21}\)
=>\(A=\frac{1}{3}-\frac{1}{21}\)
=>\(A=\frac{2}{7}\)
1/ Tính tổng : \(S=3+\frac{3}{2}+\frac{3}{2^2}+...+\frac{3}{2^9}\)
2/ Tính: \(B=\frac{10}{56}+\frac{10}{140}+\frac{10}{260}+...+\frac{10}{1400}\)
\(E=\frac{1}{25.27}+\frac{1}{27.29}+...+\frac{1}{73.75}.\)
\(F=\frac{15}{90.94}+\frac{15}{94.98}+...+\frac{15}{146.150}\)
\(G=\frac{10}{56}+\frac{10}{140}+\frac{10}{260}+...+\frac{10}{1400}\)
Bài 2
\(\frac{1}{2^2}+\frac{1}{3^2}+...+\frac{1}{100^2}.\)
CMR A<\(\frac{3}{4}\)
Tính B=\(\frac{10}{56}+\frac{10}{140}+\frac{10}{260}+.....+\frac{10}{1400}\)
Tính hợp lí(nếu có thể)
E=\(\frac{10}{56}+\frac{10}{140}+\frac{10}{260}+...+\frac{10}{1400}\)
a) Tính A=\(\frac{10}{56}+\frac{10}{140}+\frac{10}{260}+..........+\frac{10}{1400}\)
b) Tìm x thuộc Z , biết \(\frac{1.2+2.3+3.4+....+99.100}{x^2+\left(x^2+1\right)+\left(x^2+2\right)+.....+\left(x^2+99\right)}=50\frac{116}{131}\)
Tính: \(\frac{10}{56}+\frac{10}{140}+\frac{10}{260}+...+\frac{10}{1400}\)
M=\(\frac{10}{56}+\frac{10}{140}+\frac{10}{260}+.....+\frac{10}{1400}\)
tính tổng M
Tính tổng
S=\(\frac{10}{56}+\frac{10}{140}+\frac{10}{260}+...+\frac{10}{1400}\)
tính \(B=\frac{10}{56}+\frac{10}{140}+\frac{10}{260}+...+\frac{10}{1400}\)
