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1

AIEEE 2009

If the roots of the equation $$b{x^2} + cx + a = 0$$ imaginary, then for all real values of $$x$$, the expression $$3{b^2}{x^2} + 6bcx + 2{c^2}$$ is :
A
less than $$4ab$$
B
greater than $$-4ab$$
C
less than $$-4ab$$
D
greater than $$4ab$$

Explanation

Given that roots of the equation

$$b{x^2} + cx + a = 0$$ are imaginary

$$\therefore$$ $${c^2} - 4ab < 0\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,...\left( i \right)$$

Let $$y = 3{b^2}{x^2} + 6bc\,x + 2{c^2}$$

$$\Rightarrow 3{b^2}{x^2} + 6bc\,x + 2{c^2} - y = 0$$

As $$x$$ is real, $$D \ge 0$$

$$\Rightarrow 36{b^2}{c^2} - 12{b^2}\left( {2{c^2} - y} \right) \ge 0$$

$$\Rightarrow 12{b^2}\left( {3{c^2} - 2{c^2} + y} \right) \ge 0$$

$$\Rightarrow {c^2} + y \ge 0$$

$$\Rightarrow y \ge - {c^2}$$

But from eqn. $$(i),$$ $${c^2} < 4ab$$

or $$- {c^2} > - 4ab$$

$$\therefore$$ we get $$y \ge - {c^2} > - 4ab$$

$$y > - 4ab$$
2

AIEEE 2009

If $$\,\left| {z - {4 \over z}} \right| = 2,$$ then the maximum value of $$\,\left| z \right|$$ is equal to :
A
$$\sqrt 5 + 1$$
B
2
C
$$2 + \sqrt 2$$
D
$$\sqrt 3 + 1$$

Explanation

Given that $$\left| {z - {4 \over z}} \right| = 2$$

Now $$\left| z \right| = \left| {z - {4 \over z} + {4 \over { - z}}} \right| \le \left| {z - {4 \over z}} \right| + {4 \over {\left| z \right|}}$$

$$\Rightarrow \left| z \right| \le 2 + {4 \over {\left| z \right|}}$$

$$\Rightarrow {\left| z \right|^2} - 2\left| z \right| - 4 \le 0$$

$$\Rightarrow \left( {\left| z \right| - {{2 + \sqrt {20} } \over 2}} \right)\left( {\left| z \right| - {{2 - \sqrt {20} } \over 2}} \right) \le 0$$

$$\left( {\left| z \right| - \left( {1 + \sqrt 5 } \right)} \right)\left( {\left| z \right| - \left( {1 - \sqrt 5 } \right)} \right) \le 0$$

$$\Rightarrow \left( { - \sqrt 5 + 1} \right) \le \left| z \right| \le \left( {\sqrt 5 + 1} \right)$$

$$\Rightarrow {\left| z \right|_{\max }} = \sqrt 5 + 1$$
3

AIEEE 2009

The quadratic equations $${x^2} - 6x + a = 0$$ and $${x^2} - cx + 6 = 0$$ have one root in common. The other roots of the first and second equations are integers in the ratio 4 : 3. Then the common root is
A
1
B
4
C
3
D
2

Explanation

Let the roots of equation $${x^2} - 6x + a = 0$$ be $$\alpha$$

and $$4$$ $$\beta$$ and that of the equation

$${x^2} - cx + 6 = 0$$ be $$\alpha$$ and $$3\beta .$$ Then

$$\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\alpha + 4\beta = 6;\,\,\,\,\,\,\,4\alpha \beta = a$$

and $$\,\,\,\,\,\,\,\,\,\,\,\,\,\alpha + 3\beta = c;\,\,\,\,\,\,\,3\alpha \beta = 6$$

$$\Rightarrow \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,a = 8$$

$$\therefore$$ The equation becomes

$${x^2} - 6x + 8 = 0$$

$$\Rightarrow \left( {x - 2} \right)\left( {x - 4} \right) = 0$$

$$\Rightarrow$$ roots are $$2$$ and $$4$$

$$\Rightarrow \alpha = 2,\beta = 1$$

$$\therefore$$ Common root is $$2.$$
4

AIEEE 2008

STATEMENT - 1 : For every natural number $$n \ge 2,$$ $${1 \over {\sqrt 1 }} + {1 \over {\sqrt 2 }} + ........ + {1 \over {\sqrt n }} > \sqrt n .$$$STATEMENT - 2 : For every natural number $$n \ge 2,$$, $$\sqrt {n\left( {n + 1} \right)} < n + 1.$$$

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

Statements $$2$$ is $$\sqrt {n\left( {n + 1} \right)} < n + 1,n \ge 2$$

$$\Rightarrow \sqrt n < \sqrt {n + 1} ,n \ge 2$$ which is true

$$\Rightarrow \sqrt 2 < \sqrt 3 < \sqrt 4 < \sqrt 5 < - - - - - - \sqrt n$$

Now $$\sqrt 2 < \sqrt n \Rightarrow {1 \over {\sqrt 2 }} > {1 \over {\sqrt n }}$$

$$\sqrt 3 < \sqrt n \Rightarrow {1 \over {\sqrt 3 }} > {1 \over {\sqrt n }};$$

$$\sqrt n \le \sqrt n \Rightarrow {1 \over {\sqrt n }} \ge {1 \over {\sqrt n }}$$

Also $${1 \over {\sqrt 1 }} > {1 \over {\sqrt n }}$$

$$\therefore$$ Adding all, we get

$${1 \over {\sqrt 1 }} + {1 \over {\sqrt 2 }} + {1 \over {\sqrt 3 }} + ....... + {1 \over n} > {n \over {\sqrt n }} = \sqrt n$$

Hence both the statements are correct and statement $$2$$ is a correct explanation of statement $$-1.$$

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