JEE-Advanced 2015 (9/40)

Algebra Level 5

Let X X and Y Y be two arbitrary, 3 × 3 3 \times 3 , non-zero, skew-symmetric matrices and Z Z be an arbitrary, 3 × 3 3 \times 3 , non-zero , symmetric matrix. Then which of the following matrices is (are) skew-symmetric ? ( 1 ) Y 3 Z 4 Z 4 Y 3 ( 2 ) X 44 + Y 44 ( 3 ) X 4 Z 3 Z 3 X 4 ( 4 ) X 23 + Y 23 \begin{array}{c} (1) \, Y^3Z^4-Z^4Y^3 \quad \quad \quad \quad & (2) \, X^{44}+Y^{44} \\ (3) \, X^4Z^3-Z^3X^4 & (4) \, X^{23}+Y^{23} \end{array}
Note :

  • Submit your answer as the increasing order of the serial numbers of all the correct options.

  • For eg, if your answer is ( 1 ) , ( 2 ) (1),(2) , then submit 12 as the correct answer, if your answer is ( 2 ) , ( 3 ) , ( 4 ) (2),(3),(4) , then submit 234 as the correct answer.


The answer is 34.

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1 solution

Anurag Bisht
Feb 28, 2018

(C) ( X 4 Z 3 Z 3 X 4 ) T = ( X 4 Z 3 ) T ( Z 3 X 4 ) T = ( Z T ) 3 ( X T ) 4 ( X T ) 4 ( Z T ) 3 = Z 3 X 4 X 4 Z 3 = ( X 4 Z 3 Z 3 X 4 ) (X^4Z^3 - Z^3X^4)^T= (X^4Z^3)^T - (Z^3 X^4)^T = (Z^T)^3(X^T)^4 - (X^T)^4(Z^T)^3= Z^3X^4 - X^4Z^3= -(X^4Z^3 - Z^3X^4)

(D) ( X 23 + Y 23 ) T = ( X T ) 23 + ( Y T ) 23 = ( X ) 23 + ( Y ) 23 = ( X 23 + Y 23 ) (X^{23} + Y^{23})^T = (X^T)^{23} + (Y^T)^{23}=(-X)^{23}+(-Y)^{23} = -(X^{23}+Y^{23})

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