Verified answer

1\51 + 1\99 = 150\(51.99) <2\60

1\52 +1\98 <2\60

.......

1\59 +1\91 <2\60

=> S < 2.9\60 +1\60 +1\61 +... +1\90 < 18\60 + 1\ 60 +.. +1\60 =50\60 =5\6

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hay ta co

B= 1\n +1\(150-n) - 1\(n+1) -1\(149-n)

= 150.[ 1\(150n-n^2) - 1\(148n +149 -n^2)]

= 150.[149 - 2n]\{.....} ( {...} la mau so sau khi wi đồng.. e tự điền nha)

=> voi n<74 => B >0

=> 1\51 +1\99 > 1\52 +1\98 >... >1\59 +1\91

ma 1\51 +1\99 <2\60

=> S < 2.9\60 +1\60 +1\61 +.. +1\90 < 50\60=5\6