Question:
If one root the equation $2 x^{2}+k x+4=0$ is 2 , then the other root is
(a) 6
(b) −6
(c) −1
(d) 1
Solution:
Let $\alpha$ and $\beta$ be the roots of quadratic equation $2 x^{2}+k x+4=0$ in such a way that $\alpha=2$
Here, $a=2, b=k$ and,$c=4$
Then , according to question sum of the roots
$\alpha+\beta=\frac{-b}{a}$
$2+\beta=\frac{-k}{2}$
$\beta=\frac{-k}{2}-2$
$\beta=\frac{-k-4}{2}$
And the product of the roots
$\alpha \cdot \beta=\frac{c}{a}$
$=\frac{4}{2}$
$=2$
Putting the value of $\beta=\frac{-k-4}{2}$ in above
$2 \times \frac{(-k-4)}{2}=2$
$(-k-4)=2$
$k=-4-2$
$=-6$
Putting the value of $k$ in $\beta=\frac{-k-4}{2}$
$\beta=\frac{-(-6)-4}{2}$
$=\frac{6-4}{2}$
$=\frac{2}{2}$
$\beta=1$
Therefore, value of other root be $\beta=1$
Thus, the correct answer is $(d)$