Question

Tính nhanh A= 

Answer 1

`= 9 - 1/2 - 1/6 - 1/12 + ... - 1/90`

`= 9 - (1/1 - 1/2 + 1/2 - 1/3 + ... + 1/9 - 1/10)`

`= 9 - 1 + 1/10`

`= 81/10`

Answer 2

\(A=1-\dfrac{1}{2}+1-\dfrac{1}{6}+...+1-\dfrac{1}{90}\)

\(=9-\left(1-\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{1}{3}+...+\dfrac{1}{9}-\dfrac{1}{10}\right)\)

=9-1+1/10

=8+1/10=8,1