tính nhanh 1/2+-1/3+1/4+-1/5+1/6+-1/7+1/8 +1/7+-1/6+1/5+-1/4+1/3+-1/2

$\dfrac{1}{2}+\dfrac{-1}{3}+\dfrac{1}{4}+\dfrac{-1}{5}+\dfrac{1}{6}+\dfrac{-1}{7}+\dfrac{1}{8}+\dfrac{1}{7}+\dfrac{-1}{6}+\dfrac{1}{5}+\dfrac{-1}{4}+\dfrac{1}{3}+\dfrac{-1}{2}$

$=\left[\left(\dfrac{1}{2}+\dfrac{-1}{2}\right)+\left(\dfrac{-1}{3}+\dfrac{1}{3}\right)+\left(\dfrac{1}{4}+\dfrac{-1}{4}\right)+\left(\dfrac{-1}{5}+\dfrac{1}{5}\right)+\left(\dfrac{1}{6}+\dfrac{-1}{6}\right)+\left(\dfrac{-1}{7}+\dfrac{1}{7}\right)\right]+\dfrac{1}{8}$

$= [0+0+0+0+0+0]+\dfrac{1}{8}$

$=0+\dfrac{1}{8}$

$=\dfrac{1}{8}$