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### AIEEE 2008

Let $$A$$ be a square matrix all of whose entries are integers.
Then which one of the following is true?
A
If det $$A = \pm 1,$$ then $${A^{ - 1}}$$ exists but all its entries are not necessarily integers
B
If det $$A \ne \pm 1,$$ then $${A^{ - 1}}$$ exists and all its entries are non integers
C
If det $$A = \pm 1,$$ then $${A^{ - 1}}$$ exists but all its entries are integers
D
If det $$A = \pm 1,$$ then $${A^{ - 1}}$$ need not exists

## Explanation

As all entries of square matrix $$A$$ are integers, therefore all co-factors should also be integers.

If det $$A = \pm 1\,\,$$ then $${A^{ - 1}}\,\,$$ exists. Also all entries of $${A^{ - 1}}$$ are integers.
2

### AIEEE 2008

Let $$a, b, c$$ be any real numbers. Suppose that there are real numbers $$x, y, z$$ not all zero such that $$x=cy+bz,$$ $$y=az+cx,$$ and $$z=bx+ay.$$ Then $${a^2} + {b^2} + {c^2} + 2abc$$ is equal to
A
$$2$$
B
$$-1$$
C
$$0$$
D
$$1$$

## Explanation

The given equations are

$$\matrix{ { - x + cy + bz = 0} \cr {cx - y + az = 0} \cr {bx + ay - z = 0} \cr }$$

As $$x,y,z$$ are not all zero

$$\therefore$$ The above system should not have unique (zero) solution

$$\Rightarrow \Delta = 0 \Rightarrow \left| {\matrix{ { - 1} & c & b \cr c & { - 1} & a \cr b & a & { - 1} \cr } } \right| = 0$$

$$\Rightarrow - 1\left( {1 - {a^2}} \right) - c\left( { - c - ab} \right) + b\left( {ac + b} \right) = 0$$

$$\Rightarrow - 1 + {a^2} + {b^2} + {c^2} + 2abc = 0$$

$$\Rightarrow {a^2} + {b^2} + {c^2} + 2abc = 1$$
3

### AIEEE 2008

Let $$A$$ be $$a\,2 \times 2$$ matrix with real entries. Let $$I$$ be the $$2 \times 2$$ identity matrix. Denote by tr$$(A)$$, the sum of diagonal entries of $$a$$. Assume that $${a^2} = I.$$
Statement-1 : If $$A \ne I$$ and $$A \ne - I$$, then det$$(A)=-1$$
Statement- 2 : If $$A \ne I$$ and $$A \ne - I$$, then tr $$(A)$$ $$\ne 0$$.
A
statement - 1 is false, statement -2 is true
B
statement -1 is true, statement - 2 is true; statement - 2 is a correct explanation for statement - 1.
C
statement - 1 is true, statement - 2 is true; statement - 2 is not a correct explanation for statement - 1.
D
statement - 1 is true, statement - 2 is false.

## Explanation

Let $$A = \left[ {\matrix{ a & b \cr c & d \cr } } \right]$$ $$\,\,\,$$ then $${A^2} = 1$$

$$\Rightarrow {a^2} + bc = 1\,\,\,\,ab + bd = 0$$

$$ac + cd = 0\,\,\,\,bc + {d^2} = 1$$

From these four relations,

$${a^2} + bc = bc + {d^2} \Rightarrow {a^2} = {d^2}$$

and $$\,\,b\left( {a + d} \right) = 0 = c\left( {a + d} \right)$$

$$\,\,\,\,\,\,\,\,\,\,\,\,\,\, \Rightarrow a = - d$$

We can take $$a = 1,b = 0,c = 0,d = - 1$$

as one possible set of values, then

$$A = \left[ {\matrix{ 1 & 0 \cr 0 & { - 1} \cr } } \right]$$

Clearly $$A \ne I\,\,\,$$ and $$\,\,\,\,A \ne - I\,\,$$ and $$\,\,\,A = - 1$$

$$\therefore$$ $$\,\,\,\,\,$$ Statement $$1$$ is true.

Also if $$A \ne I\,\,\,\,\,tr\left( A \right) = 0$$

$$\therefore$$ $$\,\,\,\,\,$$ Statement $$2$$ is false.
4

### AIEEE 2007

Let $$A = \left| {\matrix{ 5 & {5\alpha } & \alpha \cr 0 & \alpha & {5\alpha } \cr 0 & 0 & 5 \cr } } \right|.$$ If $$\,\,\left| {{A^2}} \right| = 25,$$ then $$\,\left| \alpha \right|$$ equals
A
$$1/5$$
B
$$5$$
C
$${5^2}$$
D
$$1$$

## Explanation

$$\left| {{A^2}} \right| = 25 \Rightarrow {\left| A \right|^2} = 25$$

$$\Rightarrow {\left( {25\alpha } \right)^2} = 25 \Rightarrow \left| \alpha \right| = {1 \over 5}$$

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