Using Nature of Roots to find range of values

122 Views Asked by At

Find the range of values of $k$ for which $3x^2-4(k-x)+2$ is always positive for all real values of $x$.

I've tried simplifying, until I got to: $3x^2-4k+4x+2$.

Since it must always be positive, the discriminant, $b^2-4ac$ must be negative, ie $b^2-4ac<0$.

$b^2-4ac<0$

$(4)^2-4(3)(2-4k)<0$

$16-24+48k<0$

$48k<8$

$k<\frac 16$

I got stuck here. Is anyone able to help me continue?

EDIT: Realised that there was a typo, the discriminant should be negative, thank you Deepak.

1

There are 1 best solutions below

1
On BEST ANSWER

You seem to have already got the answer (which is simply $k < \frac 16$). There is only one additional modification I would make to your answer. That is to assert that the coefficient of $x^2$ is positive (in this case $3$). If it were negative, you would never be able to satisfy the required condition since for high enough values of $x$, the quadratic expression would always be negative.

EDIT: just noticed you wrote that the discriminant must be positive. I hope that's just a typo, and not a conceptual error. It seems to have been just a typo because you set up the inequality correctly. Anyway, remember that you need complex roots for a quadratic curve to lie wholly on one side of the $x$-axis or the other (which side depends on the sign of the leading coefficient). Complex roots means the discriminant is negative).