2010 AIME II Problem 11
Attempt Problem 11 of the 2010 AIME II below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2010 AIME II solutions, or check the answer key.
All problems are used with official legal permission of the Mathematical Association of America (MAA).
11.
Define a T-grid to be a matrix which satisfies the following two properties:
Exactly five of the entries are ’s, and the remaining four entries are ’s.
Among the eight rows, columns, and long diagonals (the long diagonals are and ), no more than one of the eight has all three entries equal.
Find the number of distinct T-grids.
Answer: 68
Small Hint:
There are placements of the ’s; subtract those where two or more of the eight lines are constant
Big Hint:
Two constant lines must be either a row/column of ’s with a parallel line of ’s, or two crossing lines of ’s, which use exactly five ’s
Solution:
There are matrices satisfying we subtract those with two or more constant lines. Two lines of ’s are impossible (they would need at least zeros), and a line of ’s and a line of ’s cannot cross, so they must be parallel rows or parallel columns; likewise two lines of ’s cannot be parallel ( ones), so they must cross, using exactly ones.
Case a line of ’s and a parallel line of ’s. There are choices for the all- row or column, for the parallel all- line, and ways to fill the remaining parallel line with two ’s and one matrices. Every perpendicular line then contains both a and a so no third constant line appears and nothing is double-counted.
Case two crossing lines of ’s and ’s elsewhere. The pair can be a row and a column (), a row or column with a diagonal (), or the two diagonals (), for matrices; one checks the four remaining ’s never form a constant line. So
Problem 11 in Other Years
1983 AIME · 1984 AIME · 1985 AIME · 1986 AIME · 1987 AIME · 1988 AIME · 1989 AIME · 1990 AIME · 1991 AIME · 1992 AIME · 1993 AIME · 1994 AIME · 1995 AIME · 1996 AIME · 1997 AIME · 1998 AIME · 1999 AIME · 2000 AIME I · 2000 AIME II · 2001 AIME I · 2001 AIME II · 2002 AIME I · 2002 AIME II · 2003 AIME I · 2003 AIME II · 2004 AIME I · 2004 AIME II · 2005 AIME I · 2005 AIME II · 2006 AIME I · 2006 AIME II · 2007 AIME I · 2007 AIME II · 2008 AIME I · 2008 AIME II · 2009 AIME I · 2009 AIME II · 2010 AIME I · 2011 AIME I · 2011 AIME II · 2012 AIME I · 2012 AIME II · 2013 AIME I · 2013 AIME II · 2014 AIME I · 2014 AIME II · 2015 AIME I · 2015 AIME II · 2016 AIME I · 2016 AIME II · 2017 AIME I · 2017 AIME II · 2018 AIME I · 2018 AIME II · 2019 AIME I · 2019 AIME II · 2020 AIME I · 2020 AIME II · 2021 AIME I · 2021 AIME II · 2022 AIME I · 2022 AIME II · 2023 AIME I · 2023 AIME II · 2024 AIME I · 2024 AIME II · 2025 AIME I · 2025 AIME II · 2026 AIME I · 2026 AIME II