When the positive integers are arranged in order,
filling in the successive diagonals of an infinite grid from top to bottom, as shown,
the integer 41 is in the (5,5) spot.
What integer would we see in the (10,10) spot if the rest of the grid were visible?
\(\text{The red entries in the main diagonal (n,n) form} \\ \text{an arithmetic series of the second order: {$1,5,13,25,41,\ldots$}}\)
\(\begin{array}{|r|r|r|r|} \hline n & (n,n ) & & \text{First difference} & \text{Second difference} \\ \hline 1 & (1,1) & 1 & \\ & & & 4 \\ 2 & (2,2) & 5 & & 4 \\ & & & 8 \\ 3 & (3,3) & 13 & & 4 \\ & & & 12 \\ 4 & (4,4) & 25 & & 4 \\ & & & 16 \\ 5 & (5,5) & 41 & & 4 \\ & & & 20 \\ 6 & (6,6) & 61 & & 4 \\ & & & 24 \\ 7 & (7,7) & 85 & & 4 \\ & & & 28 \\ 8 & (8,8) & 113 & & 4 \\ & & & 32 \\ 9 & (8,9) & 145 & & 4 \\ & & & 36 \\ 10 & (10,10) & \mathbf{181} & & 4 \\ \hline \end{array}\)