Question

If $$a, b, c$$  are in G.P., then what is the value of \[\left| {\begin{array}{*{20}{c}} a&b&{a + b}\\ b&c&{b + c}\\ {a + b}&{b + c}&0 \end{array}} \right|\,?\]

A. $$0$$  
B. $$1$$
C. $$ - 1$$
D. None of these
Answer :   $$0$$
Solution :
Since, $$a, b, c$$  are in G.P.
⇒ $$b^2 = ac$$
Explanding the determinant we get,
\[\begin{array}{l} \left| {\begin{array}{*{20}{c}} a&b&{a + b}\\ b&c&{b + c}\\ {a + b}&{b + c}&0 \end{array}} \right|\\ = a\left| {\begin{array}{*{20}{c}} c&{b + c}\\ {b + c}&0 \end{array}} \right| - b\left| {\begin{array}{*{20}{c}} b&{b + c}\\ {a + b}&0 \end{array}} \right| + \left( {a + b} \right)\left| {\begin{array}{*{20}{c}} b&c\\ {a + b}&{b + c} \end{array}} \right| \end{array}\]
$$\eqalign{ & = - a{\left( {b + c} \right)^2} + b\left( {a + b} \right)\left( {b + c} \right) + \left( {a + b} \right)\left( {{b^2} + bc - ac - bc} \right) \cr & = - a\left( {{b^2} + {c^2} + 2bc} \right) + b\left( {ab + ac + {b^2} + bc} \right) \cr & = - a{b^2} - a{c^2} - 2abc + a{b^2} + 2abc + {b^2}c\left( {\because {b^2} = ac} \right) \cr & = - a{c^2} + {b^2}c = - a{c^2} + ac \cdot c = - a{c^2} + a{c^2} = 0 \cr} $$

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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