Môn Toán Lớp 6: 1/21 + 1/28 + 1/36 +….+2/x.(x-1)=2/9

1/21 + 1/28 + 1/36 + … + 2/{ x. ( x – 1 )} = 2/9

=> 2/42 + 2/56 + 2/72 + … + 2/{ x. ( x – 1 )} = 2/9

=> 2/{ 6.7 } + 2/{7.8} + 2/{8.9} + … 2/{ ( x – 1 )x } = 2/9

=> 2( 1/{ 6.7 } + 1/{7.8} + 1/{8.9} + … 1/{ ( x – 1 )x } ) = 2/9

=> 2( 1/6 – 1/7 + 1/7 – 1/8 + 1/8 – 1/9 + … + 1/{x-1} – 1/x ) = 2/9

=> 2( 1/6 – 1/x ) = 2/9

=> 1/3 – 2/x = 2/9

=> 2/x = 1/3 – 2/9

=> 2/x = 1/9

=> x = 2 : 1/9

=> x = 18

$Vậy$ x =18