Read the following statements carefully.
( 1 ) The elements on the main diagonal of a symmetric matrix are all zero.
( 2 ) The elements on the main diagonal of a skew-symmetric matrix are all zero.
( 3 ) For any square matrix A , 2 1 ( A + A ′ ) is skew-symmetric.
( 4 ) For any square matrix A , 2 1 ( A − A ′ ) is skew-symmetric.
Which of the following statements are correct?
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Very easy Question !!( at most level-2 problem )
I agree. Statement 3 is not correct
Thanks. I have updated the answer to "Only 2, 4".
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(1) is incorrect because by definition for symmetric matrix A = A T that is a i j = a j i for all i , j .
(2) is correct because by definition for skew symmetric matrix A = − A T that is a i j = − a j i .But now consider the case where i = j = k (say).Here a k k = − a k k implying a k k = 0 for all k .
for,(3) and (4) see Aman Sharma 's answer it is the perfect way to do it.
P.S. in general any square matrix A = 0 . 5 ( A + A T ) + 0 . 5 ( A − A T ) where A + A T is symmetric and A − A T is skew symmetric.
Anuj Shikarkhane, I think all options are incorrect if you don't specify that: c h a r ( K ) = 2
How to do by instinct
(1) is wrong as a matrix ( 1 ) suffices
then , 3 and 4 can't be both correct, so is is 2 and 4
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I think statement 3 is incorrect:- For a square matrix A , 2 1 ( A + A ′ ) is a symetric matrix not skew symetric matrix:- Consider a square matrix A :- Also let:- 2 1 ( A + A ′ ) = K . . . . . . . . . ( 1 ) Taking transpose on both sides of equation (1):- 2 1 ( A + A ′ ) ′ = K ′ . . . . . . ( 2 ) Now using the properties well known properties of transpose operator:- 2 1 ( A + A ) ′ = 2 1 ( A ′ + ( A ′ ) ′ ) = 2 1 ( A + A ′ ) . . . . . ( 3 ) Now putting the value of 2 1 ( A + A ′ ) ′ from equation (3) in (2):- 2 1 ( A + A ′ ) = K ′ . . . . . ( 4 ) Clearly LHS of equations (1) and (A) are same hence there RHS must be same so:- K = K ′ Putting the value of K in above equation:- 2 1 ( A + A ′ ) = 2 1 ( A + A ′ ) ′ which is defination of symetric matrix not of skew symetric matrix So i think question may be wrong........it is also possible that i am doing something wrong ,can someone please explain why is statement 3 correct