JEE PYQ: Quadratic Equation Question 42
Question 42 - 2019 (09 Jan Shift 2)
If both the roots of the quadratic equation $x^2 - mx + 4 = 0$ are real and distinct and they lie in the interval $[1, 5]$, then $m$ lies in the interval:
(1) $(-5, -4)$
(2) $(4, 5)$
(3) $(5, 6)$
(4) $(3, 4)$
Show Answer
Answer: (2)
Solution
Given quadratic equation is: $x^2 - mx + 4 = 0$
Both the roots are real and distinct.
So, discriminant $B^2 - 4AC > 0$.
$\therefore m^2 - 4 \cdot 1 \cdot 4 > 0$
$\therefore (m-4)(m+4) > 0$
$\therefore m \in (-\infty, -4) \cup (4, \infty)$ …(i)
Since, both roots lies in $[1, 5]$
$\therefore \frac{-m}{-2} \in (1, 5)$
$\Rightarrow m \in (2, 10)$ …(ii)
And $1 \cdot (1 - m + 4) > 0 \Rightarrow m < 5$
$\therefore m \in (-\infty, 5)$ …(iii)
And $1 \cdot (25 - 5m + 4) > 0 \Rightarrow m < \frac{29}{5}$
$\therefore m \in \left(-\infty, \frac{29}{5}\right)$ …(iv)
From (i), (ii), (iii) and (iv), $m \in (4, 5)$