The number of 3 2 matrices A, which can be formed using the elements of the set \-2, -1, 0, 1, 2\ such that the sum of all the diagonal elements of ATA is 5, is
Answer
312
Solution
trace(ATA) = i,j aij2 = sum of squares of all 6 entries
the six squares (each 0, 1 or 4) must add up to 5.
Case I: five entries 1 and one entry 0
Ways = 61 25 = 6 32 = 192
Case II: one entry 2, one entry 1, four entries 0
Ways = 6 5 2 2 = 120
required number = 192 + 120 = 312
Answer: 312