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Balti Tee 2018 · Ülesanne 2

Algebra

A 100×100100 \times 100 table is given. For each k,1≤k≤100k, 1 \leq k \leq 100, the kk-th row of the table contains the numbers 1,2,…,k1,2, \ldots, k in increasing order (from left to right) but not necessarily in consecutive cells; the remaining 100−k100-k cells are filled with zeroes. Prove that there exist two columns such that the sum of the numbers in one of the columns is at least 19 times as large as the sum of the numbers in the other column.

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Observe that the sum of numbers in the first column is at most 1⋅100=1001 \cdot 100 = 100, the sum in the first and second columns is at most 1⋅100+2⋅991 \cdot 100 + 2 \cdot 99, the sum in the first, second and third columns is at most 1⋅100+2⋅99+3⋅981 \cdot 100 + 2 \cdot 99 + 3 \cdot 98, etc. But the sum of all nonzero numbers equals ∑i=1100i(101−i)\sum_{i=1}^{100} i(101 - i), therefore the sum in the columns from 3131-th to 100100-th is at least

∑i=31100i(101−i)=∑i=170i(101−i)=101∑i=170i−∑i=170i2=35⋅71(101−141/3)=70⋅27⋅71.\sum_{i=31}^{100} i(101-i) = \sum_{i=1}^{70} i(101-i) = 101 \sum_{i=1}^{70} i - \sum_{i=1}^{70} i^2 = 35 \cdot 71(101 - 141/3) = 70 \cdot 27 \cdot 71.

Therefore one of these columns has a sum at least 27⋅71=191727 \cdot 71 = 1917. Therefore the ratio of sums in this column and in the first one is more than 1919.

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