Question

[Nguyễn Quý Vương]

Tính nhanh

A = \(\dfrac{3}{2x5}+\dfrac{3}{5x8}+\dfrac{3}{8x11}+\dfrac{3}{11x14}+\dfrac{3}{14x17}+\dfrac{3}{17x20}\)

Answer 1

[Vu Thuy Linh]

A = \(\dfrac{1}{2}\) - \(\dfrac{1}{5}\) + \(\dfrac{1}{5}\) - \(\dfrac{1}{8}\) + \(\dfrac{1}{8}\) - \(\dfrac{1}{11}\) + \(\dfrac{1}{11}\) - \(\dfrac{1}{14}\) + \(\dfrac{1}{14}\) - \(\dfrac{1}{17}\) + \(\dfrac{1}{17}\) - \(\dfrac{1}{20}\)
   = \(\dfrac{1}{2}\) - \(\dfrac{1}{20}\)
   = \(\dfrac{9}{20}\)

Answer 2

[kodo sinichi]

cách làm như trên

=9/20