Let S= \A= bmatrixa & b\ c & d bmatrix : a, b, c, d \0, 1, 2, 3, 4\ and A2-4 A+3I=0 \ be a set of 2 2 matrices. Then the number of matrices in S, for which the sum of the diagonal elements is equal to 4, is :
- 20
- 17
- 21
- 19
Answer
(4) 19
Solution
By the Cayley–Hamilton theorem, every 2 2 matrix satisfies A2-(tr A)A+( A)I=0.
With tr A=a+d=4: A2-4A+( A)I=0
Subtracting the given A2-4A+3I=0: ( A-3)I=0
ad-bc=3, and conversely every such matrix lies in S.
Now count a+d=4, ad-bc=3 with entries in \0,1,2,3,4\:
• (a,d)=(0,4) or (4,0): ad=0 bc=-3, impossible.
• (a,d)=(1,3) or (3,1): ad=3 bc=0; pairs (b,c) with bc=0: 5+5-1=9 each 18
• (a,d)=(2,2): ad=4 bc=1 b=c=1: 1 matrix
Number of matrices =18+1=19
Answer: (4)