Tag: algebra

Questions Related to algebra

 For what value of
k, the matrix $A = \begin{bmatrix} 4 & 3 -k\\ 1 & 2 \end{bmatrix}$ is
not invertible?

  1. (a)$k = -5,$

  2. (b)$k=5,$

  3. (c)$k=0$

  4. (d)$k=3$


Correct Option: A

If the traces of $A, B$ are $20$ and $-8$, then the trace of $A+B$ is:

  1. $12$

  2. $-12$

  3. $28$

  4. $-28$


Correct Option: A
Explanation:

 the trace of an $n\times n$ square matrix A is defined to be the sum of the elements on the main diagonal (the diagonal from the upper left to the lower right) of A

the traces of A,B are 20 and −8, then the trace of $A+B$ is $trac(A+B)=trace(A)+trac(B)=20+-8=12$

If $A$ is a $3\times3$ skew-symmetric matrix, then the trace of $A$ is equal to

  1. $-1$

  2. $1$

  3. $|A|$

  4. $0$


Correct Option: D
Explanation:

We know that in a skew-symmetric matrix, diagonal elements are always 0.
So, its trace is 0.

If$A=\left[ \begin{matrix} 1 & -5 & 7 \ 0 & 7 & 9 \ 11 & 8 & 9 \end{matrix} \right] $ , then  trace of matrix $A$ is

  1. $17$

  2. $25$

  3. $3$

  4. $12$


Correct Option: A
Explanation:

Given, $A=\left[ \begin{matrix} 1 & -5 & 7 \ 0 & 7 & 9 \ 11 & 8 & 9 \end{matrix} \right] $
$tr(A)=sum\ of\ diagonal\ matrix$
$tr(A)=1+7+9=17$

If $\displaystyle :A= \left [ a _{ij} \right ]$ is a scalar matrix of order $\displaystyle :n\times n$ such that $\displaystyle :a _{ij}= k $ for all then trace of A is equal to

  1. $\displaystyle :nk$

  2. $\displaystyle :n+k$

  3. $\displaystyle :n/k$

  4. none of these


Correct Option: A
Explanation:

By definition of trace of a scalar matrix of order n, 
$tr(A)=a _{11}+ a _{22}+a _{33}+.....+a _{nn}$

$=k+k+....k $ (upto n times)

$tr(A)=nk$

If $\displaystyle :A= \left [ a _{ij} \right ]$ is a scalar matrix, then trace of A is

  1. $\displaystyle :\sum _{i} \sum _{i} a _{ij}$

  2. $\displaystyle :\sum _{i} a _{ij}$

  3. $: \sum _{ i } a _{ ij }\times { a } _{ ji }$

  4. None of these


Correct Option: B
Explanation:

By definition of trace of a matrix of order n, 
$tr(A)=a _{11}+ a _{22}+a _{33}+.....+a _{nn}$
$\displaystyle =: \sum _{ i=j }  { a } _{ ij } =: \sum _{ i } a _{ ij }$
Hence, option 'B' is correct.

If A is a skew-symmetric matrix, then trace of A is

  1. 1

  2. -1

  3. 0

  4. none of these


Correct Option: C
Explanation:

We know that for a skew-symmetric matrix the sum of diagonal elements is zero.
$a _{ij}=0    \forall i=j$
So,$ tr (A)=0$

If $A=\begin{bmatrix} 1 & -5 & 7 \ 0 & 7 & 9 \ 11 & 8 & 9 \end{bmatrix}$, then the value of tr $A$ is

  1. $17$

  2. $25$

  3. $3$

  4. $12$


Correct Option: A
Explanation:

$A=\begin{bmatrix} 1 & -5 & 7 \ 0 & 7 & 9 \ 11 & 8 & 9 \end{bmatrix}$
$trace(A)=$sum of diagonal elements$=1+7+9=17$

Ans: A

If $A = \left[ {{a _{ij}}} \right]$ and ${a _{ij}} = i\left( {i + j} \right)$ then trace of $A=$

  1. $\frac{{n\left( {n + 1} \right)\left( {2n + 1} \right)}}{6}$

  2. $\frac{{n\left( {n + 1} \right)\left( {2n + 1} \right)}}{3}$

  3. $\frac{{n\left( {n + 1} \right)}}{2}$

  4. $\frac{{{n^2}{{\left( {n + 1} \right)}^2}}}{4}$


Correct Option: B
Explanation:

Given $A=[a _{i,j}]$ and  $a _{i,j}=i(i+j)$ 

let the order of $A$ = $n\times n$

trace of $A$ =sum of all diagonal elements i.e.,$\sum[a _{i,j}]$ where $i=j$

$a _{1,1}=1(1+1)=2$
$a _{2,2}=2(2+2)=8$
$a _{3,3}=3(3+3)=18$
              $.$
              $.$
              $.$
$a _{n,n}=n(n+n)=2n^2$


$Trace$ $of$ $ A=$ $a _{1,1}+a _{2,2}+a _{3,3}+...........+a _{n,n}$ 
              $A=$  $2+8+18+.................+2n^2$
              $A=$  $2[1+4+9+................n^2]$

              $A=$  $2 \times [\frac{n(n+1)(2n+1)}{6}]$

              $A=$  $\frac{n(n+1)(2n+1)}{3}$

               $\therefore Opt$ $is$ $[B]$

If $tr(A)=3, tr(B)=5$, then $tr(AB)$=

  1. $15$

  2. $8$

  3. $3/5$

  4. $cannot\ say$


Correct Option: D