Question

Tính P=(-1)/20 + (-1)/30 + (-1)/42 + (-1)/56 + (-1)/72 + (-1)/90

Answer 1

-1/5*6 + (-1)/6*7+....+(-1)/9*10

=-1/5 +(-1)/10

=-3/10

Answer 2

P = \(\dfrac{-1}{20}+\dfrac{-1}{30}+\dfrac{-1}{42}+\dfrac{-1}{56}+\dfrac{-1}{72}+\dfrac{-1}{90}\)

= \(\left(\dfrac{1}{4\cdot5}+\dfrac{1}{5\cdot6}+\dfrac{1}{6\cdot7}+\dfrac{1}{7\cdot8}+\dfrac{1}{8\cdot9}+\dfrac{1}{9\cdot10}\right)\cdot\left(-1\right)\)=

\(\left(\dfrac{1}{4}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{8}+\dfrac{1}{8}-\dfrac{1}{9}+\dfrac{1}{9}-\dfrac{1}{10}\right)\cdot\left(-1\right)\)

=\(\left(\dfrac{1}{4}-\dfrac{1}{10}\right)\cdot\left(-1\right)\)

=\(\dfrac{3}{20}\cdot\left(-1\right)\)

=\(\dfrac{-3}{20}\)