Xin hay nhất nhé tus ^_^
$\dfrac{1}{15}$ `+` $\dfrac{1}{35}$ `+...+` $\dfrac{1}{x×(x+2)}$ `=` $\dfrac{1}{7}$
$\dfrac{1}{2}$`×(`$\dfrac{2}{15}$ `+` $\dfrac{2}{35}$ `+...+` $\dfrac{2}{x×(x+2)}$`)` `=` $\dfrac{1}{7}$
$\dfrac{1}{2}$`×(`$\dfrac{2}{3×5}$ `+` $\dfrac{2}{5×7}$ `+...+` $\dfrac{2}{x×(x+2)}$`)` `=` $\dfrac{1}{7}$
$\dfrac{1}{2}$`×(`$\dfrac{1}{3}$ `-` $\dfrac{1}{5}$ `+` $\dfrac{1}{5}$ `-` $\dfrac{1}{7}$ `+...+` $\dfrac{1}{x}$ `-` $\dfrac{1}{x+2}$`)` `=` $\dfrac{1}{7}$
$\dfrac{1}{2}$`×(`$\dfrac{1}{3}$ `-` $\dfrac{1}{x+2}$`)` `=` $\dfrac{1}{7}$
$\dfrac{1}{3}$ `-` $\dfrac{1}{x+2}$ `=` $\dfrac{1}{7}$ `:` $\dfrac{1}{2}$
$\dfrac{1}{3}$ `-` $\dfrac{1}{x+2}$ `=` $\dfrac{2}{7}$
$\dfrac{1}{x+2}$ `=` $\dfrac{1}{3}$ `-` $\dfrac{2}{7}$
$\dfrac{1}{x+2}$ `=` $\dfrac{1}{21}$
`⇒` `x+2=21`
`x=21-2`
`x=19`
Vậy `x=19`
