Quay lại đáp án

	2	thêm 127 ký tự trong đáp án
Ta có : $\frac{1}{21}$ + $\frac{1}{28}$ +...+ $\frac{2}{x(x+1)}$ = $\frac{2}{42}$ + $\frac{2}{56}$ +...+ $\frac{2}{x(x+1)}$ =2( $\frac{1}{6.7}$ + $\frac{1}{7.8}$ +...+ $\frac{1}{x(x+1)}$ )= 2( $\frac{1}{6}$ - $\frac{1}{x+1}$ )= $\frac{2}{9}$ $\Leftrightarrow $\frac{1}{6}$-$\frac{1}{x+1}$=$\frac{1}{9}$ \Leftrightarrow$$\frac{1}{x+1} $=$ \frac{1}{18}$$\Leftrightarrow $x+1=18$ \Leftrightarrow$x=17 Ta có : $\frac{1}{21}$ + $\frac{1}{28}$ +...+ $\frac{2}{x(x+1)}$ = $\frac{2}{42}$ + $\frac{2}{56}$ +...+ $\frac{2}{x(x+1)}$ =2( $\frac{1}{6.7}$ + $\frac{1}{7.8}$ +...+ $\frac{1}{x(x+1)}$ )= 2( $\frac{1}{6}$ - $\frac{1}{x+1}$ )= $\frac{2}{9}$ Phương trình cần giải đó Ta có : $\frac{1}{21}$ + $\frac{1}{28}$ +...+ $\frac{2}{x(x+1)}$ = $\frac{2}{42}$ + $\frac{2}{56}$ +...+ $\frac{2}{x(x+1)}$ =2( $\frac{1}{6.7}$ + $\frac{1}{7.8}$ +...+ $\frac{1}{x(x+1)}$ )= 2( $\frac{1}{6}$ - $\frac{1}{x+1}$ )= $\frac{2}{9}$ $\Leftrightarrow $\frac{1}{6}$-$\frac{1}{x+1}$=$\frac{1}{9}$ \Leftrightarrow$$\frac{1}{x+1} $=$ \frac{1}{18}$$\Leftrightarrow $x+1=18$ \Leftrightarrow$x=17

	1	tạo mới
Ta có : $\frac{1}{21}$ + $\frac{1}{28}$ +...+ $\frac{2}{x(x+1)}$ = $\frac{2}{42}$ + $\frac{2}{56}$ +...+ $\frac{2}{x(x+1)}$ =2( $\frac{1}{6.7}$ + $\frac{1}{7.8}$ +...+ $\frac{1}{x(x+1)}$ )= 2( $\frac{1}{6}$ - $\frac{1}{x+1}$ )= $\frac{2}{9}$ Phương trình cần giải đó

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