Question

If $${B^n} - A = I$$   and \[A = \left[ {\begin{array}{*{20}{c}} {26}&{26}&{18}\\ {25}&{37}&{17}\\ {52}&{39}&{50} \end{array}} \right],B = \left[ {\begin{array}{*{20}{c}} 1&4&2\\ 3&5&1\\ 7&1&6 \end{array}} \right],\]        then $$n =$$

A. 2  
B. 3
C. 4
D. 5
Answer :   2
Solution :
$$\eqalign{ & \because {B^n} - A = I \cr & \therefore {B^n} = I + A \cr} $$
\[\begin{array}{l} {B^n} = \left[ {\begin{array}{*{20}{c}} 1&0&0\\ 0&1&0\\ 0&0&1 \end{array}} \right] + \left[ {\begin{array}{*{20}{c}} {26}&{26}&{18}\\ {25}&{37}&{17}\\ {52}&{39}&{50} \end{array}} \right]\\ {B^n} = \left[ {\begin{array}{*{20}{c}} {27}&{26}&{18}\\ {25}&{38}&{17}\\ {52}&{39}&{51} \end{array}} \right]\\ {\rm{or, }}{\left[ {\begin{array}{*{20}{c}} 1&4&2\\ 3&5&1\\ 7&1&6 \end{array}} \right]^n} = \left[ {\begin{array}{*{20}{c}} {27}&{26}&{18}\\ {25}&{38}&{17}\\ {52}&{39}&{51} \end{array}} \right]\,\,\,\,\,.....\left( {\rm{i}} \right) \end{array}\]
$$\therefore n \ne 1$$
Now put $$n = 2,$$  then
\[\begin{array}{l} {B^2} = {\left[ {\begin{array}{*{20}{c}} 1&4&2\\ 3&5&1\\ 7&1&6 \end{array}} \right]^2} = \left[ {\begin{array}{*{20}{c}} 1&4&2\\ 3&5&1\\ 7&1&6 \end{array}} \right]\left[ {\begin{array}{*{20}{c}} 1&4&2\\ 3&5&1\\ 7&1&6 \end{array}} \right]\\ = \left[ {\begin{array}{*{20}{c}} {27}&{26}&{18}\\ {25}&{38}&{17}\\ {52}&{39}&{51} \end{array}} \right] \end{array}\]
Which is equal to R.H.S. of eq. (i).
∴ $$n = 2$$

Releted MCQ Question on
Algebra >> Matrices and Determinants

Releted Question 1

Consider the set $$A$$ of all determinants of order 3 with entries 0 or 1 only. Let $$B$$  be the subset of $$A$$ consisting of all determinants with value 1. Let $$C$$  be the subset of $$A$$ consisting of all determinants with value $$- 1.$$ Then

A. $$C$$ is empty
B. $$B$$  has as many elements as $$C$$
C. $$A = B \cup C$$
D. $$B$$  has twice as many elements as elements as $$C$$
Releted Question 2

If $$\omega \left( { \ne 1} \right)$$  is a cube root of unity, then
\[\left| {\begin{array}{*{20}{c}} 1&{1 + i + {\omega ^2}}&{{\omega ^2}}\\ {1 - i}&{ - 1}&{{\omega ^2} - 1}\\ { - i}&{ - i + \omega - 1}&{ - 1} \end{array}} \right|=\]

A. 0
B. 1
C. $$i$$
D. $$\omega $$
Releted Question 3

Let $$a, b, c$$  be the real numbers. Then following system of equations in $$x, y$$  and $$z$$
$$\frac{{{x^2}}}{{{a^2}}} + \frac{{{y^2}}}{{{b^2}}} - \frac{{{z^2}}}{{{c^2}}} = 1,$$    $$\frac{{{x^2}}}{{{a^2}}} - \frac{{{y^2}}}{{{b^2}}} + \frac{{{z^2}}}{{{c^2}}} = 1,$$    $$ - \frac{{{x^2}}}{{{a^2}}} + \frac{{{y^2}}}{{{b^2}}} + \frac{{{z^2}}}{{{c^2}}} = 1$$     has

A. no solution
B. unique solution
C. infinitely many solutions
D. finitely many solutions
Releted Question 4

If $$A$$ and $$B$$ are square matrices of equal degree, then which one is correct among the followings?

A. $$A + B = B + A$$
B. $$A + B = A - B$$
C. $$A - B = B - A$$
D. $$AB=BA$$

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Matrices and Determinants


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