What is the fifth number in line 10? What is the fifth number in line 200? one 2、3 4、5、6 7、8、9、10 11、12、13、14、15 ……

Suppose that the current row is n, and the last number of the row is 1 + 2 +. + n = (n * (n + 1)) / 2, so the last number of the ninth row is (9 * 10) / 2 = 45, the fifth number of the tenth row is 45 + 5 = 50. Confirm the position of 200. Confirm the position of 200 in the first row, that is, the smallest integer n with (n * (n + 1)) / 2 > 200, and get n = 20, that is, 200 in the 20th row

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