Tính tổng sau C= ( 1 - 1/2 ) + ( 1 - 1/6 ) + ( 1 - 1/12 ) + (1-1/20)

$C$ = $($ $1$ - $\dfrac{1}{2}$ $)$ + $($ $1$ - $\dfrac{1}{6}$ $)$ + $($ $1$ - $\dfrac{1}{12}$ $)$ + $($ $1$ - $\dfrac{1}{20}$ $)$

= $($ $1$ + $1$ +$1$ + $1$ $)$ - $($ $\dfrac{1}{2}$ + $\dfrac{1}{6}$ + $\dfrac{1}{12}$ + $\dfrac{1}{20}$ $)$

= $4$ - $($ $\dfrac{1}{1.2}$ + $\dfrac{1}{2.3}$ + $\dfrac{1}{3.4}$ + $\dfrac{1}{4.5}$ $)$

= $4$ - $($ $1$ - $\dfrac{1}{2}$ + $\dfrac{1}{2}$ - $\dfrac{1}{3}$ + $\dfrac{1}{3}$ - $\dfrac{1}{4}$ + $\dfrac{1}{4}$  - $\dfrac{1}{5}$ $)$

= $5$ - $($ $1$ - $\dfrac{1}{5}$ $)$ = $5$ - $1$ + $\dfrac{1}{5}$ = $4$ + $\dfrac{1}{5}$ 

= $\dfrac{21}{5}$