Đặt $Q = 1/21 + 1/28 + 1/36 + ... + 2/x(x+1) = 2/9$
$⇒ 1/2 . Q = 1/42 + 1/56 + 1/72 + ... + 1/x(x+1) = 1/9$
$1/6.7 + 1/7.8 + 1/8.9 + ... + 1/x(x+1) = 1/9$
$1/6 - 1/7 + 1/7 - 1/8 + 1/8 - 1/9 + ... + 1/x - 1/x+1 = 1/9$
$1/6 - 1/x+1 = 1/9$
$1/x+1 = 1/6 - 1/9$
$1/x+1 = 1/18$
$⇒ x+1 = 18$
$⇒ x = 17$
Vậy $x = 17.$
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