Maths-
General
Easy

Question

Assertion : There are only finitely many 2 × 2 matrices which commute with the matrix open square brackets table row 1 2 row cell negative 1 end cell cell negative 1 end cell end table close square brackets
Reason : If A is non-singular then it commutes with I, adj A and A–1

  1. If both (A) and (R) are true, and (R) is the correct explanation of (A).    
  2. If both (A) and (R) are true but (R) is not the correct explanation of (A).    
  3. If (A) is true but (R) is false.    
  4. If (A) is false but (R) is true.    

The correct answer is: If (A) is false but (R) is true.


    The reason R is true since
    AI = IA, AA–1 = A–1A = I, A|adj A| = |adj. A|A
    But a matrix can commute with general order matrices which may be infinite in number.
    Let B = open square brackets table row a b row c d end table close square bracketsbe a matrix which commute with A then AB = BA
    not stretchy rightwards double arrowopen square brackets table row 1 2 row cell negative 1 end cell cell negative 1 end cell end table close square bracketsopen square brackets table row a b row c d end table close square brackets= open square brackets table row 1 2 row cell negative 1 end cell cell negative 1 end cell end table close square brackets
    = open square brackets table row cell a plus 2 c end cell cell b plus 2 d end cell row cell negative a minus c end cell cell negative b minus d end cell end table close square brackets = open square brackets table row cell a minus b end cell cell 2 a minus b end cell row cell c minus d end cell cell 2 c minus d end cell end table close square brackets
    not stretchy rightwards double arrowa + 2c = a – b, b + 2d = 2a – b, – a – c= c – d, – b – d = 2c – d
    The above four relations are equivalent to only two independent relations
    a – d = b, b + 2c = 0
    If d = lambda, then a = b + lambda = –2c + lambda
    Thus, open square brackets table row cell lambda minus 2 c end cell cell negative 2 c end cell row c lambda end table close square brackets are all possible 2 × 2 matrices which commute with given matrix A = open square brackets table row 1 2 row cell negative 1 end cell cell negative 1 end cell end table close square brackets
    lambda and c being any arbitrary complex numbers. Thus assertion is therefore false.

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