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1

AIEEE 2005

If both the roots of the quadratic equation $${x^2} - 2kx + {k^2} + k - 5 = 0$$ are less than 5, then $$k$$ lies in the interval
A
$$\left( {5,6} \right]$$
B
$$\left( {6,\,\infty } \right)$$
C
$$\left( { - \infty ,\,4} \right)$$
D
$$\left[ {4,\,5} \right]$$

Explanation

both roots are less than $$5,$$

then $$(i)$$ Discriminant $$\ge 0$$

$$\left( {ii} \right)\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,p\left( 5 \right) > 0$$

$$\left( {iii} \right)\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,{{Sum\,\,of\,\,roots} \over 2} < 5$$

Hence $$\left( i \right)\,\,\,\,\,\,4{k^2} - 4\left( {{k^2} + k - 5} \right) \ge 0$$

$$4{k^2} - 4{k^2} - 4k + 20 \ge 0$$

$$4k \le 20 \Rightarrow k \le 5$$

$$\left( {ii} \right)\,\,\,\,\,f\left( 5 \right) > 0;25 - 10k + {k^2} + k - 5 > 0$$

or $${k^2} - 9k + 20 > 0$$

or $$k\left( {k - 4} \right) - 5\left( {k - 4} \right) > 0$$

or $$\left( {k - 5} \right)\left( {k - 4} \right) > 0$$

$$\Rightarrow k \in \left( { - \infty ,4} \right) \cup \left( { - \infty ,5} \right)$$

$$\left( {iii} \right)\,\,\,\,\,\,{{Sum\,\,of\,\,roots} \over 2}$$

$$= - {b \over {2a}} = {{2k} \over 2} < 5$$

The intersection of $$(i)$$, $$(ii)$$ & $$(iii)$$ gives

$$k \in \left( { - \infty ,4} \right).$$
2

AIEEE 2005

In a triangle $$PQR,\;\;\angle R = {\pi \over 2}.\,\,If\,\,\tan \,\left( {{P \over 2}} \right)$$ and $$\tan \left( {{Q \over 2}} \right)$$ are the roots of $$a{x^2} + bx + c = 0,\,\,a \ne 0$$ then
A
$$a = b + c$$
B
$$c = a + b$$
C
$$b = c$$
D
$$b = a + c$$

Explanation

$$\angle$$R = 90o $$\therefore$$ $$\angle$$P + $$\angle$$Q = 90o

$$\Rightarrow$$ $${P \over 2} + {Q \over 2} = {{90} \over 2} = 45$$o

$$\tan \left( {{P \over 2}} \right),\tan \left( {{Q \over 2}} \right)$$ are the roots of $$a{x^2} + bx + c = 0$$

$$\therefore$$ $$\tan \left( {{P \over 2}} \right) + \tan \left( {{Q \over 2}} \right) = - {b \over a},\,\,$$

and $$\tan \left( {{P \over 2}} \right).\tan \left( {{Q \over 2}} \right) = {c \over a}$$

$${{\tan \left( {{P \over 2}} \right) + \tan \left( {{Q \over 2}} \right)} \over {1 - \tan \left( {{P \over 2}} \right)\tan \left( {{Q \over 2}} \right)}}$$

$$= \tan \left( {{P \over 2} + {Q \over 2}} \right)$$= tan 45o = 1

$$\Rightarrow {{ - {b \over a}} \over {1 - {c \over a}}} = 1$$

$$\Rightarrow - {b \over a} = {a \over a} - {c \over a}$$

$$\Rightarrow - b = a - c$$ or $$c = a + b.$$
3

AIEEE 2004

If one root of the equation $${x^2} + px + 12 = 0$$ is 4, while the equation $${x^2} + px + q = 0$$ has equal roots,
then the value of $$'q'$$ is
A
4
B
12
C
3
D
$${{49} \over 4}$$

Explanation

$$4$$ is a root of $${x^2} + px + 12 = 0$$

$$\Rightarrow 16 + 4p + 12 = 0$$

$$\Rightarrow p = - 7$$

Now, the equation $${x^2} + px + q = 0$$

has equal roots.

$$\therefore$$ $${p^2} - 4q = 0$$ $$\Rightarrow q = {{{p^2}} \over 4} = {{49} \over 4}$$
4

AIEEE 2004

If $$\left( {1 - p} \right)$$ is a root of quadratic equation $${x^2} + px + \left( {1 - p} \right) = 0$$ then its root are
A
$$- 1,2$$
B
$$- 1,1$$
C
$$0,-1$$
D
$$0,1$$

Explanation

Let the second root be $$\alpha .$$

Then $$\alpha + \left( {1 - p} \right) = - p \Rightarrow \alpha = - 1$$

Also $$\alpha .\left( {1 - p} \right) = 1 - p$$

$$\Rightarrow \left( {\alpha - 1} \right)\left( {1 - p} \right) = 0$$

$$\Rightarrow p = 1$$ [as $$\alpha = - 1$$]

$$\therefore$$ Roots are $$\alpha = - 1$$ and $$p-1=0$$

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