Maths Olympiad Prep

Track / Stage 3 / 204 of 260 #204 of 1964

Problem 204

AMC 10/12, early questions
Number theory Difficulty 3.7 Find the answer

What number is directly above 142142 in this array of numbers?
123456789101112\begin{array}{cccccc}& & & 1 & &\\ & & 2 & 3 & 4 &\\ & 5 & 6 & 7 & 8 & 9\\ 10 & 11 & 12 &\cdots & &\\ \end{array}

Pick one

Official solution

Solution 1
Notice that a number in row kk is 2k2k less than the number directly below it. For example, 55, which is in row 33, is (2)(3)=6(2)(3)=6 less than the number below it, 1111.
From row 1 to row kk, there are k(1+(1+2k)2)=k2k \left(\frac{1+(-1+2k)}{2} \right) = k^2 numbers in those kk rows. Because there are 122=14412^2=144 numbers up to the 12th row, 142142 is in the kthk^{th} row. The number directly above is in the 11th row, and is 2222 less than 142142. Thus the number directly above 142142 is 14222=(C) 120142-22=\boxed{\text{(C)}\ 120}.

Solution 2
Writing a couple more rows, the last number in each row ends in a perfect square. Thus 142142 is two left from the last number in its row, 144144. One left and one up from 144144 is the last number of its row, also a perfect square, and is 121121. This is one right and one up from 142142, so the number directly above 142142 is one less than 121121, or (C) 120\boxed{\text{(C)}\ 120}.
Remark: The last number in each row ends in a perfect square because there is an odd amount of numbers in each row and the sum of the first n odd numbers is always a perfect square.

Solution 3 (eyeball it)
We can notice that even numbers are above even numbers so we can narrow our answers down to 2 solutions, 120120 and 122122. Notice each number on the right end of each row is a perfect square. The perfect square closest to these is 121121 so 122122 is the first number on the next row. Notice 142142 is the third to last number on its row so it is too far away from 122, so our answer is (C) 120\boxed{\text{(C)}\ 120}.

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.