\(A=\frac{2.3.5+4.9.25+6.9.35+10.21.40}{2.3.7+4.9.35+6.9.49+10.21.56}\)

\(A=\frac{\left(2.3.5\right)+\left(2.3.5\right).2.3.5+\left(2.3.5\right).3.3.7+\left(2.3.5\right).5.7.8}{\left(2.3.7\right)+\left(2.3.7\right).2.3.5+\left(2.3.7\right).3.3.7+\left(2.3.7\right).5.7.8}\)

\(A=\frac{\left(2.3.5\right).\left(1+2.3.5+3.3.7+5.7.8\right)}{\left(2.3.7\right).\left(1+2.3.5+3.3.7+5.7.8\right)}\)

\(A=\frac{2.3.5}{2.3.7}=\frac{5}{7}.\)

\(B=\left(-\frac{3}{4}\right).\left(-\frac{8}{9}\right).\left(-\frac{15}{16}\right)...\left(-\frac{399}{400}\right)\)

\(B=-\frac{1.3.2.4.3.5...19.21}{2.2.3.3.4.4...20.20}\)

\(B=-\frac{1.2.3...19.3.4.5...21}{2.3.4...20.2.3.4...20}=-\frac{21}{40}.\)

A=5.(2.3+4.9.5+6.9.7+10.21.8)/7.(2.3+4.9.5+6.9.7+10.21.8)

A=5/7

A = 5/7

=95136

mk mới học lớp 5 

Ta có : \(A=\frac{2.3.5.1.1.1+2.3.5.2.3.5+2.3.5.3.3.7+2.3.5.5.7.8}{2.3.7.1.1.1+2.3.7.2.3.5+2.3.7.3.3.7+2.3.7.5.7.8}\)

\(=\frac{2.3.5\left(1.1.1+2.3.5+3.3.7+5.7.8\right)}{2.3.7\left(1.1.1+2.3.5+3.3.7+5.7.8\right)}=\frac{2.3.5}{2.3.7}=\frac{5}{7}\)

thôi thôi

A = \(\dfrac{2.3.5+4.9.35+6.9.35+10.21.40}{2.3.7+4.9.35+6.9.49+10.21.56}\)

A = \(\dfrac{2.3.5+\left(2.3.5\right).2.3.5+\left(2.3.5\right).3.3.7+\left(2.3.5\right).5.7.8}{2.3.7+\left(2.3.7\right).2.3.5+\left(2.3.7\right).3.3.7+\left(2.3.7\right).5.7.8}\)

A = \(\dfrac{2.3.5.\left(1+2.3.5+3.3.7+5.7.8\right)}{2.3.7\left(1+2.3.5+3.3.7+5.7.8\right)}\)

A = \(\dfrac{2.3.5}{2.3.7}\)

A = \(\dfrac{5}{7}\)

Ta có: \(A=\dfrac{2\cdot3\cdot5+4\cdot9\cdot35+6\cdot9\cdot35+10\cdot21\cdot40}{2\cdot3\cdot7+4\cdot9\cdot35+6\cdot9\cdot49+10\cdot21\cdot56}\)

\(=\dfrac{2\cdot3\cdot5+2\cdot3\cdot5\cdot6\cdot7+2\cdot3\cdot5\cdot3\cdot3\cdot7+2\cdot3\cdot5\cdot5\cdot7\cdot8}{2\cdot3\cdot7+2\cdot3\cdot7\cdot2\cdot3\cdot5+2\cdot3\cdot7\cdot3\cdot3\cdot7+2\cdot3\cdot7\cdot5\cdot7\cdot8}\)

\(=\dfrac{2\cdot3\cdot5\cdot\left(1+2\cdot3\cdot7+3\cdot3\cdot7+5\cdot7\cdot8\right)}{2\cdot3\cdot7\cdot\left(1+2\cdot3\cdot5+3\cdot3\cdot7+5\cdot7\cdot8\right)}\)

\(=\dfrac{5}{7}\cdot\dfrac{386}{374}=\dfrac{965}{1309}\)