đáp án là 277.1937

277.1937

giúp mình với ạ

\(\frac{4}{1x3x5}+\frac{4}{3x5x7}+...+\frac{4}{9x11x13}\)

\(=\frac{1}{1x3}-\frac{1}{3x5}+\frac{1}{3x5}-...+\frac{1}{9x11}-\frac{1}{11x13}\)

\(=\frac{1}{3}-\frac{1}{143}\)

\(=\frac{140}{429}\)

4/1x3x5+4/3x5x7+4/5x7x9+4/7x9x11+4/9x11x13

=1/1x3-1/3x5+1/3x5-1/5x7+1/5x7-1/7x9+1/7x9-1/9x11+1/9x11-1/11x13

=1/1x3-1/11x13

=1/3-1/143=140/429

\(\dfrac{4}{1\times3\times5}+\dfrac{4}{3\times5\times7}+\dfrac{4}{5\times7\times9}+\dfrac{4}{7\times9\times11}\)

=\(\dfrac{5-1}{1\times3\times5}+\dfrac{7-3}{3\times5\times7}+\dfrac{9-5}{5\times7\times9}+\dfrac{11-7}{7\times9\times11}\)

=\(\dfrac{1}{1\times3}-\dfrac{1}{3\times5}+\dfrac{1}{3\times5}-\dfrac{1}{5\times7}+...+\dfrac{1}{9\times11}-\dfrac{1}{11\times13}\)

=\(\dfrac{1}{3}-\dfrac{1}{143}=\dfrac{140}{429}\)

A = \(\dfrac{4}{1\times3\times5}\) + \(\dfrac{4}{3\times5\times7}\) +\(\dfrac{4}{5\times7\times9}\) + \(\dfrac{4}{7\times9\times11}\) + \(\dfrac{4}{9\times11\times13}\)

A = \(\dfrac{1}{1\times3}\)-\(\dfrac{1}{3\times5}\)+\(\dfrac{1}{3\times5}\)-\(\dfrac{1}{5\times7}\)+...+\(\dfrac{1}{9\times11}\)-\(\dfrac{1}{11\times13}\)

A = \(\dfrac{1}{1\times3}\) - \(\dfrac{1}{11\times13}\)

A = \(\dfrac{1}{3}-\dfrac{1}{143}\)

A = \(\dfrac{140}{429}\)

Bài 2:

A = \(\dfrac{1991}{1990}\) x \(\dfrac{1992}{1991}\) x \(\dfrac{1993}{1992}\) x \(\dfrac{1994}{1993}\) x \(\dfrac{1995}{997}\)

A = \(\dfrac{1994\times1995}{1990\times997}\)

 A = \(\dfrac{997\times2\times5\times399}{5\times2\times199\times997}\)

A = \(\dfrac{399}{199}\)

tui hok tính đc đó!