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 = \(\dfrac{399}{199}\)
