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Page 1 Edurev123 3 Matrix 3.1 Find a hermitian and a skew-hermitian matrix each whose sum is the matrix [ ?? ?? ?? -?? ?? ?? +?? ?? ?? -?? +?? ?? ?? ?? ] (2009 : 12 Marks) Solution: Given any matrix ?? we can write it as ?? = 1 2 (?? +?? ? )+ 1 2 (?? -?? * ) where ?? * is the complex tranjugate of ?? . Also 1 2 (?? +?? * ) is always hermitian as [ 1 2 (?? +?? * )] * = 1 2 (?? +?? * ) * = 1 2 (?? * +?? ) = 1 2 (?? +?? * ) And 1 2 (?? +?? * ) is skew hermitian as [ 1 2 (?? -?? * )]* = 1 2 (?? * -?? )=-[ 1 2 (?? -?? * )] ? ?? = 1 2 [?? +?? * ]+ 1 2 [?? -?? * ] =?? +?? where ?? is hermitian and ?? skew hermitian. Taking ?? as given matrix. ?? = 1 2 (?? +?? * ) = 1 2 {[ 2?? 3 -1 1 2+3?? 2 -?? +1 4 5?? ]+[ -2?? ?? ?? +1 3 2-3?? 4 -1 2 -5?? ]} =[ 0 2 ??/2 2 2 3 -??/2 3 0 ] which is hermitian. ?????? ?? = 1 2 (?? -?? * ) Page 2 Edurev123 3 Matrix 3.1 Find a hermitian and a skew-hermitian matrix each whose sum is the matrix [ ?? ?? ?? -?? ?? ?? +?? ?? ?? -?? +?? ?? ?? ?? ] (2009 : 12 Marks) Solution: Given any matrix ?? we can write it as ?? = 1 2 (?? +?? ? )+ 1 2 (?? -?? * ) where ?? * is the complex tranjugate of ?? . Also 1 2 (?? +?? * ) is always hermitian as [ 1 2 (?? +?? * )] * = 1 2 (?? +?? * ) * = 1 2 (?? * +?? ) = 1 2 (?? +?? * ) And 1 2 (?? +?? * ) is skew hermitian as [ 1 2 (?? -?? * )]* = 1 2 (?? * -?? )=-[ 1 2 (?? -?? * )] ? ?? = 1 2 [?? +?? * ]+ 1 2 [?? -?? * ] =?? +?? where ?? is hermitian and ?? skew hermitian. Taking ?? as given matrix. ?? = 1 2 (?? +?? * ) = 1 2 {[ 2?? 3 -1 1 2+3?? 2 -?? +1 4 5?? ]+[ -2?? ?? ?? +1 3 2-3?? 4 -1 2 -5?? ]} =[ 0 2 ??/2 2 2 3 -??/2 3 0 ] which is hermitian. ?????? ?? = 1 2 (?? -?? * ) = 1 2 {[ 2?? 3 -1 1 2+3?? 2 -?? +1 4 5?? ]-[ -2?? ?? ?? +1 3 2-3?? 4 -1 2 -5?? ]} = [ 2?? 1 -(?? +2) 2 -1 3?? -1 (-?? +2) 2 1 5?? ] So, ?? and ?? are required vector where ?? =?? +?? and ?? is Hermitian and ?? skew Hermitian. 3.2 Find a ?? ×?? real matrix ?? which is both orthogonal and skew symmetric. Can there exist a ?? ×?? real matrix which is both orthogonal and skaw symmetric. Justify your answer. (2009 : 20 Marks) Solution: Approach : Consider the form of a skew symmetric matrix (diagonal elements zero) and impose conditions for orthogonality. Let ?? be a 2×2 skew symmetric matrix and ?? =[ ?? ?? ?? ?? ]. ?? is skew symmetric ? ?? =-?? ?? => [ ?? ?? ?? ?? ]=[ -?? -?? -?? -?? ]??? =?? =0 and ?? =-?? ? ?? =[ 0 ?? -?? 0 ] If ?? is orthogonal then ?? ?? ? =?? => ?? 2 =1??? =±?? ?[ 0 1 -1 0 ] and [ 0 -1 1 0 ] are the only matrices that are orthogonal and skew symmetric. Again let ?? be a 3×3 skew symmetric matrix. Then ?? =[ 0 ?? ?? -?? 0 ?? -?? -?? 0 ] as seen in previous case. Also if ?? is orthogonal. Page 3 Edurev123 3 Matrix 3.1 Find a hermitian and a skew-hermitian matrix each whose sum is the matrix [ ?? ?? ?? -?? ?? ?? +?? ?? ?? -?? +?? ?? ?? ?? ] (2009 : 12 Marks) Solution: Given any matrix ?? we can write it as ?? = 1 2 (?? +?? ? )+ 1 2 (?? -?? * ) where ?? * is the complex tranjugate of ?? . Also 1 2 (?? +?? * ) is always hermitian as [ 1 2 (?? +?? * )] * = 1 2 (?? +?? * ) * = 1 2 (?? * +?? ) = 1 2 (?? +?? * ) And 1 2 (?? +?? * ) is skew hermitian as [ 1 2 (?? -?? * )]* = 1 2 (?? * -?? )=-[ 1 2 (?? -?? * )] ? ?? = 1 2 [?? +?? * ]+ 1 2 [?? -?? * ] =?? +?? where ?? is hermitian and ?? skew hermitian. Taking ?? as given matrix. ?? = 1 2 (?? +?? * ) = 1 2 {[ 2?? 3 -1 1 2+3?? 2 -?? +1 4 5?? ]+[ -2?? ?? ?? +1 3 2-3?? 4 -1 2 -5?? ]} =[ 0 2 ??/2 2 2 3 -??/2 3 0 ] which is hermitian. ?????? ?? = 1 2 (?? -?? * ) = 1 2 {[ 2?? 3 -1 1 2+3?? 2 -?? +1 4 5?? ]-[ -2?? ?? ?? +1 3 2-3?? 4 -1 2 -5?? ]} = [ 2?? 1 -(?? +2) 2 -1 3?? -1 (-?? +2) 2 1 5?? ] So, ?? and ?? are required vector where ?? =?? +?? and ?? is Hermitian and ?? skew Hermitian. 3.2 Find a ?? ×?? real matrix ?? which is both orthogonal and skew symmetric. Can there exist a ?? ×?? real matrix which is both orthogonal and skaw symmetric. Justify your answer. (2009 : 20 Marks) Solution: Approach : Consider the form of a skew symmetric matrix (diagonal elements zero) and impose conditions for orthogonality. Let ?? be a 2×2 skew symmetric matrix and ?? =[ ?? ?? ?? ?? ]. ?? is skew symmetric ? ?? =-?? ?? => [ ?? ?? ?? ?? ]=[ -?? -?? -?? -?? ]??? =?? =0 and ?? =-?? ? ?? =[ 0 ?? -?? 0 ] If ?? is orthogonal then ?? ?? ? =?? => ?? 2 =1??? =±?? ?[ 0 1 -1 0 ] and [ 0 -1 1 0 ] are the only matrices that are orthogonal and skew symmetric. Again let ?? be a 3×3 skew symmetric matrix. Then ?? =[ 0 ?? ?? -?? 0 ?? -?? -?? 0 ] as seen in previous case. Also if ?? is orthogonal. ? ???? ? =?? ? [ 0 ?? ?? -?? 0 ?? -?? -?? 0 ][ 0 -?? -?? ?? 0 -?? ?? ?? 0 ] =[ 1 0 0 0 1 0 0 0 1 ] ? [ ?? 2 +?? 2 ???? -???? ???? ?? 2 +?? 2 ???? -???? ???? ?? 2 -?? 2 ] =[ 1 0 0 0 1 0 0 0 1 ] ? ?? 2 +?? 2 =?? 2 +?? 2 =?? 2 +?? 2 =1 …(i) =???? =???? =???? =0 …(ii) From (ii) two of ?? ,?? ,?? must be zero if ?? =?? =0??? 2 +?? 2 =0?1. Similarly in other cases it can be shown the system of equations is not compatible. So, a 3×3 skew symmetric matrix can not be orthogonal. 3.3. If ?? ?? ,?? ?? ,?? ?? are eigen values of the matrix ?? =[ ?? ?? -?? ?? ?? ???? ?? ?? ?? ???? ] Show that v?? ?? ?? +?? ?? ?? +?? ?? ?? =v???????? (2010 : 12 Marks) Solution: ?????????? ?? h?? ???????????? ?? =[ 26 -2 2 2 21 4 4 2 28 ] Now, finding eigen values of ?? Page 4 Edurev123 3 Matrix 3.1 Find a hermitian and a skew-hermitian matrix each whose sum is the matrix [ ?? ?? ?? -?? ?? ?? +?? ?? ?? -?? +?? ?? ?? ?? ] (2009 : 12 Marks) Solution: Given any matrix ?? we can write it as ?? = 1 2 (?? +?? ? )+ 1 2 (?? -?? * ) where ?? * is the complex tranjugate of ?? . Also 1 2 (?? +?? * ) is always hermitian as [ 1 2 (?? +?? * )] * = 1 2 (?? +?? * ) * = 1 2 (?? * +?? ) = 1 2 (?? +?? * ) And 1 2 (?? +?? * ) is skew hermitian as [ 1 2 (?? -?? * )]* = 1 2 (?? * -?? )=-[ 1 2 (?? -?? * )] ? ?? = 1 2 [?? +?? * ]+ 1 2 [?? -?? * ] =?? +?? where ?? is hermitian and ?? skew hermitian. Taking ?? as given matrix. ?? = 1 2 (?? +?? * ) = 1 2 {[ 2?? 3 -1 1 2+3?? 2 -?? +1 4 5?? ]+[ -2?? ?? ?? +1 3 2-3?? 4 -1 2 -5?? ]} =[ 0 2 ??/2 2 2 3 -??/2 3 0 ] which is hermitian. ?????? ?? = 1 2 (?? -?? * ) = 1 2 {[ 2?? 3 -1 1 2+3?? 2 -?? +1 4 5?? ]-[ -2?? ?? ?? +1 3 2-3?? 4 -1 2 -5?? ]} = [ 2?? 1 -(?? +2) 2 -1 3?? -1 (-?? +2) 2 1 5?? ] So, ?? and ?? are required vector where ?? =?? +?? and ?? is Hermitian and ?? skew Hermitian. 3.2 Find a ?? ×?? real matrix ?? which is both orthogonal and skew symmetric. Can there exist a ?? ×?? real matrix which is both orthogonal and skaw symmetric. Justify your answer. (2009 : 20 Marks) Solution: Approach : Consider the form of a skew symmetric matrix (diagonal elements zero) and impose conditions for orthogonality. Let ?? be a 2×2 skew symmetric matrix and ?? =[ ?? ?? ?? ?? ]. ?? is skew symmetric ? ?? =-?? ?? => [ ?? ?? ?? ?? ]=[ -?? -?? -?? -?? ]??? =?? =0 and ?? =-?? ? ?? =[ 0 ?? -?? 0 ] If ?? is orthogonal then ?? ?? ? =?? => ?? 2 =1??? =±?? ?[ 0 1 -1 0 ] and [ 0 -1 1 0 ] are the only matrices that are orthogonal and skew symmetric. Again let ?? be a 3×3 skew symmetric matrix. Then ?? =[ 0 ?? ?? -?? 0 ?? -?? -?? 0 ] as seen in previous case. Also if ?? is orthogonal. ? ???? ? =?? ? [ 0 ?? ?? -?? 0 ?? -?? -?? 0 ][ 0 -?? -?? ?? 0 -?? ?? ?? 0 ] =[ 1 0 0 0 1 0 0 0 1 ] ? [ ?? 2 +?? 2 ???? -???? ???? ?? 2 +?? 2 ???? -???? ???? ?? 2 -?? 2 ] =[ 1 0 0 0 1 0 0 0 1 ] ? ?? 2 +?? 2 =?? 2 +?? 2 =?? 2 +?? 2 =1 …(i) =???? =???? =???? =0 …(ii) From (ii) two of ?? ,?? ,?? must be zero if ?? =?? =0??? 2 +?? 2 =0?1. Similarly in other cases it can be shown the system of equations is not compatible. So, a 3×3 skew symmetric matrix can not be orthogonal. 3.3. If ?? ?? ,?? ?? ,?? ?? are eigen values of the matrix ?? =[ ?? ?? -?? ?? ?? ???? ?? ?? ?? ???? ] Show that v?? ?? ?? +?? ?? ?? +?? ?? ?? =v???????? (2010 : 12 Marks) Solution: ?????????? ?? h?? ???????????? ?? =[ 26 -2 2 2 21 4 4 2 28 ] Now, finding eigen values of ?? [ 26-?? -2 2 2 21-?? 4 4 2 28-?? ]=0 ? (26-?? )[(21-?? )(28-?? )-8]+2[56-2?? -16]+2[4-84+4?? )=0 ? (26-?? )[588-49?? +?? 2 -8]+2[40-2?? ]+2[4?? -80]=0 ? (26-?? )[580-49?? +?? 2 ]+80-4?? +8?? -160=0 ? 15080-1274?? +26?? 2 -580?? +49?? 2 -?? 3 -80+4?? =0 ? 15000-1850?? +75?? 2 -?? 3 =0 ? ?? 3 -75?? 2 +??850?? -??5000=0 ?????? ?? 1 ,?? 2 ,?? 3 ???? ?????????? ???? ???? .(1) ? ?? 1 +?? 2 +?? 3 =75 ?? 1 ?? 2 +?? 2 ?? 3 +?? 3 ?? 4 =1850 ?? 1 ?? 2 ?? 3 =15000 ???????? , (?? 1 +?? 2 +?? 3 ) 2 =?? 2 1 +?? 2 2 +?? 3 2 +2(?? 1 ?? 2 +?? 2 ?? 3 +?? 3 ?? 1 ) Putting above values in above eqn., we get (75) 2 =?? 1 2 +?? 2 2 +?? 3 2 +2(1850) ? ?? 1 2 +?? 2 2 +?? 3 2 =5625-3700 ? ?? 1 2 +?? 2 2 +?? 3 2 =1925=1949 ? v?? 1 2 +?? 2 2 +?? 3 2 =v1949 3.4 Let ?? and ?? be ?? ×?? matrices over reals. Show that ?? -???? is invertible if ?? - ???? is invertible. Deduce that ???? and ???? have same eigen values. (2010 : 20 Marks) Solution: Given ?? and ?? are two ?? ×?? matrices over reels. Let ?? =(?? -???? ) -1 (Assuming that ?? -AB is invertible) ? ?? =?? +???? +(???? ) 2 +(???? ) 3 +?.. (By binomial expansion) ?????? , ?? ×?? =???? +(???? ) 2 +(???? ) 3 +? ? ?? ×?? ×?? =?? +???? +(???? ) 2 +(???? ) 3 +? =(?? -???? ) -1 ?(?? -???? ) is invertible if ?? -???? is invertible. To show that ???? and ???? have same eigen values. Let ?? be an eigen-value of ???? , Case 1 : if ?? =0, then ?????? =0.?? (?? - ?????????? ???????????? ) => (0.?? -???? )?? =0 Page 5 Edurev123 3 Matrix 3.1 Find a hermitian and a skew-hermitian matrix each whose sum is the matrix [ ?? ?? ?? -?? ?? ?? +?? ?? ?? -?? +?? ?? ?? ?? ] (2009 : 12 Marks) Solution: Given any matrix ?? we can write it as ?? = 1 2 (?? +?? ? )+ 1 2 (?? -?? * ) where ?? * is the complex tranjugate of ?? . Also 1 2 (?? +?? * ) is always hermitian as [ 1 2 (?? +?? * )] * = 1 2 (?? +?? * ) * = 1 2 (?? * +?? ) = 1 2 (?? +?? * ) And 1 2 (?? +?? * ) is skew hermitian as [ 1 2 (?? -?? * )]* = 1 2 (?? * -?? )=-[ 1 2 (?? -?? * )] ? ?? = 1 2 [?? +?? * ]+ 1 2 [?? -?? * ] =?? +?? where ?? is hermitian and ?? skew hermitian. Taking ?? as given matrix. ?? = 1 2 (?? +?? * ) = 1 2 {[ 2?? 3 -1 1 2+3?? 2 -?? +1 4 5?? ]+[ -2?? ?? ?? +1 3 2-3?? 4 -1 2 -5?? ]} =[ 0 2 ??/2 2 2 3 -??/2 3 0 ] which is hermitian. ?????? ?? = 1 2 (?? -?? * ) = 1 2 {[ 2?? 3 -1 1 2+3?? 2 -?? +1 4 5?? ]-[ -2?? ?? ?? +1 3 2-3?? 4 -1 2 -5?? ]} = [ 2?? 1 -(?? +2) 2 -1 3?? -1 (-?? +2) 2 1 5?? ] So, ?? and ?? are required vector where ?? =?? +?? and ?? is Hermitian and ?? skew Hermitian. 3.2 Find a ?? ×?? real matrix ?? which is both orthogonal and skew symmetric. Can there exist a ?? ×?? real matrix which is both orthogonal and skaw symmetric. Justify your answer. (2009 : 20 Marks) Solution: Approach : Consider the form of a skew symmetric matrix (diagonal elements zero) and impose conditions for orthogonality. Let ?? be a 2×2 skew symmetric matrix and ?? =[ ?? ?? ?? ?? ]. ?? is skew symmetric ? ?? =-?? ?? => [ ?? ?? ?? ?? ]=[ -?? -?? -?? -?? ]??? =?? =0 and ?? =-?? ? ?? =[ 0 ?? -?? 0 ] If ?? is orthogonal then ?? ?? ? =?? => ?? 2 =1??? =±?? ?[ 0 1 -1 0 ] and [ 0 -1 1 0 ] are the only matrices that are orthogonal and skew symmetric. Again let ?? be a 3×3 skew symmetric matrix. Then ?? =[ 0 ?? ?? -?? 0 ?? -?? -?? 0 ] as seen in previous case. Also if ?? is orthogonal. ? ???? ? =?? ? [ 0 ?? ?? -?? 0 ?? -?? -?? 0 ][ 0 -?? -?? ?? 0 -?? ?? ?? 0 ] =[ 1 0 0 0 1 0 0 0 1 ] ? [ ?? 2 +?? 2 ???? -???? ???? ?? 2 +?? 2 ???? -???? ???? ?? 2 -?? 2 ] =[ 1 0 0 0 1 0 0 0 1 ] ? ?? 2 +?? 2 =?? 2 +?? 2 =?? 2 +?? 2 =1 …(i) =???? =???? =???? =0 …(ii) From (ii) two of ?? ,?? ,?? must be zero if ?? =?? =0??? 2 +?? 2 =0?1. Similarly in other cases it can be shown the system of equations is not compatible. So, a 3×3 skew symmetric matrix can not be orthogonal. 3.3. If ?? ?? ,?? ?? ,?? ?? are eigen values of the matrix ?? =[ ?? ?? -?? ?? ?? ???? ?? ?? ?? ???? ] Show that v?? ?? ?? +?? ?? ?? +?? ?? ?? =v???????? (2010 : 12 Marks) Solution: ?????????? ?? h?? ???????????? ?? =[ 26 -2 2 2 21 4 4 2 28 ] Now, finding eigen values of ?? [ 26-?? -2 2 2 21-?? 4 4 2 28-?? ]=0 ? (26-?? )[(21-?? )(28-?? )-8]+2[56-2?? -16]+2[4-84+4?? )=0 ? (26-?? )[588-49?? +?? 2 -8]+2[40-2?? ]+2[4?? -80]=0 ? (26-?? )[580-49?? +?? 2 ]+80-4?? +8?? -160=0 ? 15080-1274?? +26?? 2 -580?? +49?? 2 -?? 3 -80+4?? =0 ? 15000-1850?? +75?? 2 -?? 3 =0 ? ?? 3 -75?? 2 +??850?? -??5000=0 ?????? ?? 1 ,?? 2 ,?? 3 ???? ?????????? ???? ???? .(1) ? ?? 1 +?? 2 +?? 3 =75 ?? 1 ?? 2 +?? 2 ?? 3 +?? 3 ?? 4 =1850 ?? 1 ?? 2 ?? 3 =15000 ???????? , (?? 1 +?? 2 +?? 3 ) 2 =?? 2 1 +?? 2 2 +?? 3 2 +2(?? 1 ?? 2 +?? 2 ?? 3 +?? 3 ?? 1 ) Putting above values in above eqn., we get (75) 2 =?? 1 2 +?? 2 2 +?? 3 2 +2(1850) ? ?? 1 2 +?? 2 2 +?? 3 2 =5625-3700 ? ?? 1 2 +?? 2 2 +?? 3 2 =1925=1949 ? v?? 1 2 +?? 2 2 +?? 3 2 =v1949 3.4 Let ?? and ?? be ?? ×?? matrices over reals. Show that ?? -???? is invertible if ?? - ???? is invertible. Deduce that ???? and ???? have same eigen values. (2010 : 20 Marks) Solution: Given ?? and ?? are two ?? ×?? matrices over reels. Let ?? =(?? -???? ) -1 (Assuming that ?? -AB is invertible) ? ?? =?? +???? +(???? ) 2 +(???? ) 3 +?.. (By binomial expansion) ?????? , ?? ×?? =???? +(???? ) 2 +(???? ) 3 +? ? ?? ×?? ×?? =?? +???? +(???? ) 2 +(???? ) 3 +? =(?? -???? ) -1 ?(?? -???? ) is invertible if ?? -???? is invertible. To show that ???? and ???? have same eigen values. Let ?? be an eigen-value of ???? , Case 1 : if ?? =0, then ?????? =0.?? (?? - ?????????? ???????????? ) => (0.?? -???? )?? =0 ? (0.?? -???? )?? =0 ? 0 =|0.?? -???? |=|-?? ||?? |=|?? ||-?? | =|0.?? -???? | ?0 is an eigen-value of ???? aiso. Case 2 : If ?? ?0. Let ?? is not an eigen-value of ???? . Then ?????? ????? ?(???? -???? )?? ?0 ? |???? -???? |?0??? ?? | 1 ?? ???? -1| ?0 ? | ???? ?? -?? |?0?| ?? ?? ·?? -?? |?0 or |?? - ?? ?? ·?? |?0??? - ?? ?? ·?? is invertihle. ? By abc e deduction, it can be concluded that ?? -?? · ?? ?? is also invertible. ? |?? -?? · ?? ?? |?0 ? ?? ?? |?? - ?? -?? ?? |?0 ? |???? -???? |?0 ??? is not an eigen-value of ???? which contradicts our assumption. ??? is an eigen-value of ???? also. ????? and ???? have same eigen-values. 3.5 Let ?? be a non-singular, ?? ×?? square matrix. Show that ?? ( adj. ?? )=|?? |.?? ?? . Hence, show that ladj. (Adj. ?? )|=|?? | (?? -?? ) ?? . (2011: 10 Marks) Solution: If ?? be any ?? -rowed square matrix, then (adj.?? )?? =?? (adj.?? )=|?? |?? ?? (??) where ?? ?? is a unit matrix of order ?? and adj. ?? is adjoint of the matrix ?? . Replacing ?? by adj. ?? in (i), we get ( adj. A)(adj. adj. A) =|adj.A|?? ??Read More
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