Question

If $$\alpha ,\beta $$  are non-real numbers satisfying $${x^3} - 1 = 0$$   then the value of \[\left| {\begin{array}{*{20}{c}} {\lambda + 1}&\alpha &\beta \\ \alpha &{\lambda + \beta }&1 \\ \beta &1&{\lambda + \alpha } \end{array}} \right|\]     is equal to

A. $$0$$
B. $${\lambda ^3}$$  
C. $${\lambda ^3} + 1$$
D. None of these
Answer :   $${\lambda ^3}$$
Solution :
\[\vartriangle = \left( {\lambda + 1 + \alpha + \beta } \right)\left| {\begin{array}{*{20}{c}} 1&1&1 \\ \alpha &{\lambda + \beta }&1 \\ \beta &1&{\lambda + \alpha } \end{array}} \right|\,\,\left( {{\text{Using }}{R_1} \to {R_1} + {R_2} + {R_3}} \right)\]
\[\vartriangle = \lambda \left| {\begin{array}{*{20}{c}} 1&0&1 \\ \alpha &{\lambda + \beta - \alpha }&{1 - \alpha } \\ \beta &{1 - \beta }&{\lambda + \alpha - \beta } \end{array}} \right| = \lambda \left| {\begin{array}{*{20}{c}} {\lambda + \beta - \alpha }&{1 - \alpha } \\ {1 - \beta }&{\lambda + \alpha - \beta } \end{array}} \right|\]
$$\vartriangle = \lambda \left\{ {{\lambda ^2} - {{\left( {\alpha - \beta } \right)}^2} - \left( {1 - \alpha } \right)\left( {1 - \beta } \right)} \right\}$$
$$\vartriangle = {\lambda ^3} - \lambda \left\{ {{\alpha ^2} + {\beta ^2} - 2\alpha \beta + 1 - \alpha - \beta + \alpha \beta } \right\} = 0\,\,{\text{because }}{\alpha ^2} = \beta ,{\beta ^2} = \alpha ,\alpha \beta = 1.$$

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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