Question:
If $\alpha, \beta$ are the zeroes of $k x^{2}-2 x+3 k$ such that $\alpha+\beta=\alpha \beta$, then $k=?$
(a) $\frac{1}{3}$
(b) $\frac{-1}{3}$
(c) $\frac{2}{3}$
(d) $\frac{-2}{3}$
Solution:
(c) $\frac{2}{3}$
Here, $\mathrm{p}(x)=x^{2}-2 x+3 k$
Comparing the given polynomial with $a x^{2}+b x+c$, we get:
$a=1, b=-2$ and $c=3 k$
It is given that $\alpha$ and $\beta$ are the roots of the polynomial.
$\therefore \alpha+\beta=-\frac{b}{a}$
$=>\alpha+\beta=-\left(\frac{-2}{1}\right)$
$=>\alpha+\beta=2 \quad \ldots(\mathrm{i})$
Also, $\alpha \beta=\frac{c}{a}$
$=>\alpha \beta=\frac{3 k}{1}$
$=>\alpha \beta=3 k \quad \ldots($ ii $)$
Now, $\alpha+\beta=\alpha \beta$
$=>2=3 k \quad[\mathrm{Using}(\mathrm{i})$ and $(\mathrm{ii})]$
$=>k=\frac{2}{3}$