Let A be a 3 3 real matrix such that A pmatrix1\\0\\1 pmatrix=2 pmatrix1\\0\\1 pmatrix, A pmatrix-1\\0\\1 pmatrix=4 pmatrix-1\\0\\1 pmatrix, A pmatrix0\\1\\0 pmatrix=2 pmatrix0\\1\\0 pmatrix. Then, the system (A-3I) pmatrixx\ \ pmatrix= pmatrix1\\2\\3 pmatrix has :
- exactly two solutions
- infinitely many solutions
- unique solution
- no solution
Answer
(3) unique solution
Solution
Let C1, C2, C3 be the columns of A (A ej=Cj).
C1+C3= pmatrix2\\0\\2 pmatrix, -C1+C3= pmatrix-4\\0\\4 pmatrix, C2= pmatrix0\\2\\0 pmatrix
C1= pmatrix3\\0\\-1 pmatrix, C3= pmatrix-1\\0\\3 pmatrix
A-3I= pmatrix0&0&-1\\0&-1&0\\-1&0&0 pmatrix, |A-3I|=1 0
∵ |A-3I| 0, the system has exactly one solution: z=-1, y=-2, x=-3
Answer: (3)