Ta có:
\(C=\frac{2}{3.5}+\frac{2}{5.7}+\frac{2}{7.9}+...+\frac{2}{19.21}\)
\(=\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+...+\frac{1}{19}-\frac{1}{21}\)
\(=\frac{1}{3}-\frac{1}{21}=\frac{7}{21}-\frac{1}{21}=\frac{6}{21}=\frac{2}{7}\)
các bạn.
A=\(\frac{2}{15}+\frac{2}{35}+\frac{2}{63}+...+\frac{2}{399}\)
\(\frac{2}{15}+\frac{2}{35}+\frac{2}{63}+...+\frac{2}{399}=?\)
\(\frac{2}{15}\)+\(\frac{2}{35}\)+\(\frac{2}{63}\)+...+\(\frac{2}{99}\)+\(\frac{2}{399}\)
A=\(\frac{2}{15}\)+\(\frac{2}{35}\)+\(\frac{2}{63}\)+...+\(\frac{2}{399}\)
Tìm x thuộc N
\(x-\frac{20}{11\cdot13}-\frac{20}{13\cdot15}-\frac{20}{15\cdot17}-...-\frac{20}{53\cdot55}=\frac{3}{11}\)
\(x+\frac{2}{35}+\frac{2}{63}+...+\frac{2}{399}=\frac{2}{7}\)
tính tổng
A=\(\frac{1}{10}\)+\(\frac{1}{15}\)+\(\frac{1}{21}\)+........+\(\frac{1}{120}\)
B=\(\frac{2}{15}\)+\(\frac{2}{35}\)+\(\frac{2}{63}\)+........+\(\frac{2}{399}\)
Bài 1: Tính bằng cách hợp lí nhất.
a.\(\frac{1}{1.2}+\frac{1}{2.3}+...+\frac{1}{2006.2007}\)
b.\(\frac{30}{51}-\frac{20}{52}+\frac{14}{34}-\frac{56}{91}-2\)
c.\([\frac{1}{3}+\frac{12}{67}+\frac{13}{41}]-[\frac{79}{67}-\frac{28}{41}]\)
d.\(\frac{2}{15}+\frac{2}{35}+\frac{2}{63}+\frac{2}{99}+...+\frac{2}{399}\)
Ai nhanh mik tick 3 cái, mik dg cần gấp
Tính nhanh A = \(\frac{2}{3}+\frac{2}{15}+\frac{2}{35}+...+\frac{2}{399}\)
\(\frac{4^2}{15}.\frac{5^2}{24}.\frac{6^2}{35}.....\frac{20^2}{399}\) tính nhanh
