Áp dụng theo dạng toán số ai cập ta có:
4/1.5+4/5.9+4/9.13+4/13.17+4/17.21=1/1-1/5+1/5-1/9+1/9-1/13+1/13-1/17+1/17-1/21=1-1/21 < 1
Vậy tổng đó < 1
Tinh nhanh:
\(\frac{7}{1\cdot5}+\frac{7}{5\cdot9}+\frac{7}{9\cdot13}+\frac{7}{13\cdot17}+\frac{7}{17\cdot21}\)
\(\frac{2}{4\cdot7}-\frac{2}{5\cdot9}+\frac{2}{7\cdot10}-\frac{2}{9\cdot13}+\frac{2}{10\cdot13}-\frac{2}{13\cdot17}+...+\frac{2}{301\cdot304}-\frac{2}{401\cdot405}CM:tich,tren,< \frac{1}{15}\)
Tính \(21\cdot\left(\frac{4}{1\cdot3}-\frac{8}{3\cdot5}+\frac{12}{5\cdot7}-\frac{16}{7\cdot9}+...+\frac{36}{17\cdot19}-\frac{40}{19\cdot21}\right)\)
Rút gọn :\(\frac{2\cdot3\cdot5+4\cdot9\cdot25+6\cdot9\cdot35+10\cdot21\cdot40}{2\cdot3\cdot5+4\cdot9\cdot35+6\cdot9\cdot49+10\cdot21\cdot56}\)
Tìm x:
\(\frac{\left(13\frac{2}{9}-15\frac{2}{3}\right)\cdot\left(30^2-5^4\right)}{\left(18\frac{3}{7}-17\frac{1}{4}\right)\cdot\left(25-12\cdot5^2\right)}\cdot x=\frac{\frac{2}{11}+\frac{3}{13}+\frac{4}{15}+\frac{5}{17}}{4\frac{1}{11}+\frac{5}{13}+\frac{9}{15}+\frac{13}{17}}\)
Rút gọn:
a/ \(\frac{4\cdot7}{9\cdot32}\)
b/ \(\frac{3\cdot21}{14\cdot15}\)
c/ \(\frac{2\cdot5\cdot13}{26\cdot35}\)
d/ \(\frac{9\cdot6-9\cdot3}{18}\)
e/ \(\frac{17\cdot5-17}{3-20}\)
f/ \(\frac{49+7\cdot49}{49}\)
Tính \(B=\frac{1}{10\cdot9}+\frac{1}{18\cdot13}+\frac{1}{26\cdot17}+.............+\frac{1}{802\cdot405}\)
Các bạn giải giúp mình nhé! Thank you !
A = \(\frac{4}{1\cdot3}+\frac{4}{3\cdot5}+\frac{4}{5\cdot7}+...+\frac{4}{11\cdot13}\)
\(\frac{2}{11\cdot13}+\frac{2}{13\cdot15}+...+\frac{2}{19\cdot21}-x+4+\frac{221}{231}=\frac{7}{3}\)
ai nhanh tick ( giải đầy đủ nha )
