1/2x3 + 1/3x4 + 1/4x5 + 1/5x6 +.....+1/20x21=x/14 cứu mÌnh vỚI

`1/(2xx3) + 1/(3xx4) + 1/(4xx5) + 1/(5xx6) +.....+1/(20xx21)=x/14`

`1/2 - 1/3 + 1/3 -1/4 + 1/4 - 1/5 + 1/5 -1/6 +...+ 1/20 - 1/21 = x/14`

`1/2 - 1/21 = x/14`

`21/42 - 2/42 = x/14`

`19/42 =x /14`

`x =(14xx19)/42`

`x = 19/3`

$\dfrac{1}{2 × 3}$ `+` $\dfrac{1}{3 × 4}$ `+` $\dfrac{1}{4 × 5}$ `+` $\dfrac{1}{5 × 6}$  `+` `...` `+` $\dfrac{1}{20 × 21}$ `=` $\dfrac{x}{14}$ 

`=` $\dfrac{1}{2}$  `-` $\dfrac{1}{3}$ `+` $\dfrac{1}{3}$ `-` $\dfrac{1}{4}$ `+` $\dfrac{1}{4}$   `-` $\dfrac{1}{5}$  `+` $\dfrac{1}{5}$ `-` $\dfrac{1}{6}$ `+` `...` `+` $\dfrac{1}{20}$  `-` $\dfrac{1}{21}$ `=` $\dfrac{x}{14}$  

`=` $\dfrac{1}{2}$  `-` $\dfrac{1}{21}$  `=` $\dfrac{x}{14}$

`=` $\dfrac{21}{42}$  `-` $\dfrac{2}{42}$ `=` $\dfrac{x}{14}$ $\text{( quy đồng mẫu số )}$

`=` $\dfrac{19}{42}$ `=` $\dfrac{x}{14}$

`=` `x`                       `=` $\dfrac{14 × 19}{42}$ 

`=` `x`                       `=` $\dfrac{19}{3}$ 

$#Zen$